AFATL-TR-72-401__Developement of 20MM and 30MM Plastic-Aluminium Cartridge Cases [1972]

7/28/2019 AFATL-TR-72-401__Developement of 20MM and 30MM Plastic-Aluminium Cartridge Cases [1972]

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AFATL-TR-72-201

DEVELOPMENT OF 20MM AND 30MM PLASTIC/ALUMINUM

CARTRIDGE CASES

AAI CORPORATION

TECHNICAL REPORT AFATL-TR-72-201

OCTOBER 1972 1 ) C

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Distr ibut ion l imi ted to U . S . Government agenc ies only ; th i s repor t document s test and evaluat ion; d is t r ibu t ion l imi t a t ion app l i ed c tober 1 9 7 2 . t h e r r eques t s for th i s document must be r e f e r r ed to the Air Force Armament Labora to ry D L D G ) , Egl in Air Force Base, Flor ida 3 2 5 4 2 .

AIR ORCE RMAMENT ABORATORY AIR PORCI SYSTEMS COMMAND UNITED STATES AIR FORCE

E6LIN AIR FORCE BASE, FLORIDA


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FORKWORD

This repor t documents work on th e 2 0 mm an d 30mm Plast it*/Aluminum Cartridge cases, performed during the per iod March 1970 t o ' J September li 72 by AAI Corporat ion, Cockeysvi 1 l e , Maryland, under Contract F08()Ji>-70-C 0067 with th e Air Force Armament Laboratory, K g I i n Ai r Forte Base, Florida. The Project Kngineers for the Armament Laboratory were Mr . David (',. brig (DLDG) and Major Stephen J . Bilsburv (DLDG).

The report has been assigned AAI Corporation l.nginee ring Report Number KR- 7 2 5 0 .

Ulis technical report has been reviewed an d i s p p r Vl'U .

DE M. DAVTS Director, Guns an d Rockets Division

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ABSTRACT

Basic feasibil i ty was established for th e 2 0 r n n and 30mm plas t ic /a luminum car t r idge cases under this program. The case design consists of a plastic body joined mechanical ly to an aluminum base forming a composite assembly. Work on th e 30mm case was l imited to Mann barrel fi rings while th e 20mm case was fi red successfu l ly i n both a Mann barrel and th e M61 automatic gu n at a rate of 4 3 0 0 rounds per minute. Test f i r ings of the 2 0 m n t case i n the M30 gun, however, were unsuccessful . The effor t on th e 2 0 m m case progressed into a development program including high and low temperature firing; in th e M61. Sat isfac to ry case per fo rmance ha s been established at th e temperature extremes

(160F an d - 6 5 F ' l an d ambient temperatures with th e case design being developed as f ar . s post le within th e scope of this contract .

Distr ibution l imited to U . S . Government agencies only; t i i i s report documents test and evaluation; distr ibution l imitation applied October 1972 . Other requests for t h i s document must be referred to th e Air Force Armament Laboratory (DLDC), Eglin Air Force Base, Florida 32 54 2 .

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Section

I

II

II I

IV

Appendixes

I

II

III

IV

V

VI

TABLE OF CONTENTS

Ti t l e

INTRODUCTION

DESIGN DESCRIPTIONS t . 20mm Cartridge Case

a. hysical Descript ion b. erformance

2. 30mm ("artridge Case

a . hysical Descript ion b. erformance

TEST ETHODS

DEVELOPMENTAL TESTING 1. 0mm Cartridge ase 2. 0m m Cartridge ase

CONCLUSIONS AND ECOMMENDATIONS

20MM ARTRIDGE CASE DRAWINGS

30MM ARTRIDGE ASE DRAWINGS

20MM OLD RAWINGS

30MM OLD DRAWINGS

MANUFACTURER'S DDRESSES

RECORD F TEST FIRINGS tS RELATED O OL D HANGES FO R 20MM PLASTIC/ALUMINUM CASE

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LIST OF FIGURES

Figure

1 2 3

4 5 6

7 8

10 1 1 1 2 1 3 1 4 1 5 1 6 1 7 1 8

Title ' ' a g e

20mm Plastic'Aluminum Cartridge Gases 20mm Plastic/Aluminum Cartridge Case Munition Oscilloscope Readouts of Pressure Versus Time Curves for 20mm Cartridge Cases 30mm Plast i c / Aluminum Cartridge Gase 0 30mm Plast i e/Aluminum Cartridge Case Munition 1 Oscilloscope Readout o f Pressure Versus Time Curve

for 30mm Plastic/Aluminum Cartridge Case 1

30mm Mann barrel

5 Oscilloscope Readouts of Pressure Versus Time

Curves for 20mm Cartridge Cases

8

Oscilloscope Readout of Timing Trace for M 6 1 Cannon Firing a t 4300 Rounds per Minute 1 Case Wall Reinforcing to Resist Link S q u e e / . e 2 Mold Gating at Start of Development 4 Improved Mold Gating 5 Typical Case Failures a t Low Temperatures 8 Base Erosion Failure on J i ' m m Plastic /Aluminum . ' a s o 4

Mold Changes at Start of Modified Contract 6 Locking Buttress Modification 7 20mm Case Wall Modification 3 Dimensions Inspected o n 20mm Plastic/Aluminum Case .... 4

LIST OF TABLES

Table

itle

I Results of Firings of CT L Propeltants i n Brass M103 Cases

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II

ypes of Sealants and Points o f Application

II I isting of Types of Plastic Materials Tested

IV iring Data Recorded During Testing of 30mm

Plastic/Aluminum Case

VI-1 Test Firings as Related to Mold Changes for 20mm Plastic/Aluminum Cartridge Case

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VI

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SECTION I

INTRODUCTION

The pr imary object ive of this program was to establish basic feas ib i l i ty for plas t ic /a luminum car t r idge cases in 20mm and 30mm sizes. Th e 20mm cart r idge case was to be pat terned a f te r th e brass Ml03 case to be compat ib le with exist ing weapons and to eventual ly serve as a replacement for th e M103 case. he 20mm case was test f i red in th e M6 1 cannon and i n a Mann barrel . The 30mm car t r idge case i s a ne w design which was evolved to achieve certain performance requirements. No weapon for this munit ion current ly exists i n the inventory; therefore, tes t ing was accomplished i n a specially constructed Mann barrel .

At th e request of th e AFATL project engineer, a short burst of 20mm plas t ic /a luminum case rounds were fi red i n th e M39 gun. Those tests were unsuccessful as th e cases were damaged by th e sharp, intermittent feed notion of th e gun system. Those f ew rounds that were chambered intact were fi red an d extracted properly.

The 2 0 mmand 30mm plas t ic /a luminum car t r idge cases ar e both of th e same basic des ign which consists of an a luminum base joined to a plastic body. The us e of these mater ia ls represents a sizeable weight reduct ion over a comparable case of brass. The standard Ml03 brass 20mm cartridge case i s over three times as heavy as th e plas t ic /a luminum case developed i n this effort . The weight di fference between th e assembled munit ions ut i l iz ing brass an d plas t ic /a luminum cases i s 0.1J36 lb pe r round. When th e number of rounds carried by various aircraf t i s cons idered, the pay load weight savings i s signif icant. The fo l lowing list gives typical examples of the weight savings that may be expected with various a i rcraf t using 20mm ammunition:

Aircraf t Numb r of Rounds

2 0 8 4

We ight Saved

F- l l l 38 8 l b

F - 4 E 63 8 1 19 l b

F -105 1200 2 23 lb

A C - 1 3 0 Gunsh p 6000 1116 lb A - 7 D 1000 1 8 6 lb

In addit ion to weight improvement, a significant advantage i s realized with th e use of plas t ic /a luminum cases in that a crit ical and expensive material (brass) s replaced by non-cr i t ical and inexpensive materials .

The main program objective, estab l i shment of basic feas ib i l i ty, was sat is f ied early in th e program with th e successful test f i r ing of th e 0 n u i i case in th e Mann barrel and th e 2 0 mm case in th e M61 cannon at ambient

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t e m p e r a t u r e . T h e r e f o r e , th e m a j o r i t y of t ' i e e f f o r t wa s c o n c e n t r a t e d n r e f i n e m e n t o f th e d e s i g n a n d , in p a r t i c u l a r, t h e a c h i e v e m e n t of p r o p e r p e r - f o r m a n c e of th e 2 0 m m c a s e a t t e m p e r a t u r e e x t r e m e s n th e M61 c a n n o n .

I n c l u d e d in t h i s r e p o r t s a d e s c r i p t i o n of e a c h o f th e c a r t r i d g e c a s e s , e x p l a n a t i o n of t e s t m e t h o d s , a n a r r a t i v e d e s c r i b i n g th e d e v e l o p m e n t a l t e s t i n g , c o n c l u s i o n s an d r e c o m m e n d a t i o n s , a n d a p p e n d i x e s w h i c h c o n t a i n d rawi ngs of th e m o l d s a n d c a s e c o m p o n e n t H e t a i l s .

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SECTION II

DESIGN DESCRIPTIONS

1 . 0MM CARTRIDGE CASE a. PHYSICAL DESCRIPTION

The 20mm car t r idge case i s a two-piece compos i te consist ing of a plastic body an d an a luminum base. he tw o pieces ar e median ica l ly joined by mat ing but t ress-shaped teeth which snap into place when th e plastic body

is pressed

into the base. The plastic body extends

inside

the base an d insulates th e th in-wal led aluminum section from contact with th e ho t propellant gases. Figure shows th e 20mm plas t ic /a luminum case.

The final design of th e 20mm case joint contains tw o but t resses for mechanica l ly joining th e plastic body an d th e a luminum base. The molded buttresses of th e plas t ic body are a press f it of 0 . 0 1 0 to 0 . 015 inch on th e diameter with th e corresponding surfaces in th e base. t assembly, th e plastic body i s pressed into th e base an d th e teeth snap into place. The f it on th e plast ic provides a f i rm joint that resists rotational s l ippage and forms a watert ight seal . n f ir ing, th e plastic inside th e aluminum base obturates by internal ga s pressure agains t th e base, thus seal ing the joint area from ga s leakage. The joint des ign al lows some relative movement between th e tw o parts in a lengthwise direction. If a longitudinal compressive force i s appl ied to th e case, there will be some rearward movement of th e plastic body inside th e a luminum base. The case i s made approximate ly 0 . 010 inch longer than standard so that at chambering th e case will be compressed an d shortened by that amount. t f ir ing, th e bolt deflec t ion that occurs at peak pressure ca n then be taken up by that amount of compression befo re an y tensile loads are appl ied to th e joint area. This serves to reduce the stress levels in th e case that may occur from axial tension caused by bolt deflec t ion an d decreases th e possibi l i ty of failure at the joint.

The base of th e car t r idge case i s subjected to th e most severe internal stresses of any part of th e case. The extractor groove area i s unsuppor ted by th e chamber an d must withstand th e stresses caused by internal ga s pressure. Tests have shown 7 0 7 5 - T 6 a luminum a l loy to be th e mater ia l best suited for this purpose since i t has th e required mechanica l proper t ies to withstand th e high stresses imposed at firing. In addition, th e low density of aluminum makes i t attract ive f rom a weight standpoint. detai l print of th e car t r idge base, AAI Drawing No. 5 35 - 4 0001, i s presented i n Appendix of this report.

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Figure 1. 20mm Plastic/Aluminum artridge a^es

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The final design of th e base contains tw o buttresses corresponding to those on th e plastic body for mechanical ly locking th e tw o components. The thin walls of th e base expand outward at firing to contact th e chamber. The outside dimensions ar e such that a l ine- to - l ine contact with th e chamber will exist at th e minimum chamber an d maximum base size tolerances. At th e opposite dimensional tolerances, maximum chamber and minimum base, a clearance of 0 .004 inch on th e diameter exists . Hi e material elongation i s suff ic ien t to permit base expansion at f ir ing to contact the chamber walls .

The base will accept th e standard M52A3B1 electr ic primer used i n th e M103 brass cartr idge case.

To prevent corrosion, a chemical fi lm coating pe r MIL-C-55M i s recommended for the aluminum base. I t i s a good electrical conductor so that th e operation of th e electr ic primer will no t be impaired. The chemical f i lm coating ca n be dyed any color should color coding of th e munit ion be desired.

The plastic body i s an inject ion molded part of glass- re in forced nylon. The formulation consists of type 1 2 nylon resin mixed with 0 .25 inch long glass fibers to the amount of 5 0 percent by weight of th e total mix. Th e material i s marketed by Thermofil , Inc. , Ypsilanti, Michigan. The base resin i s obtained through Henley & Co. Inc. , New York, N.Y. from Hls of West Germany. Thermofil mixes the base resin with th e desired glass loading.

The outer dimensions on th e plastic body provide a l ine-to- l i n e contact with th e chamber at minimum chamber and maximum case tolerances an d a 0 . 004- inch c learance on the diameter at th e opposite extremes. The joint area diameter i s molded 0 .0 1 0 to 0 .0 1 5 - in c h oversize to provide f or a tight f it with base in order to prevent rotational slippage an d to effec t a water t ight seal. The seal ha d been demonstrated to be watert ight by a 7 2 - hour immersion test.

The wall thickness i s 0.062 inch forward near the necked-down area of the case. This thickness provides suff ic ien t strength to resist t h e - squeeze from th e M14 link.

Projecti le re tent ion is accomplished with a continuous, circumferent ia l bead that i s molded into th e inside surface of th e case neck. Th e bead mates

with th e cr imp groove in th e M 5 5 A2 projecti le . At assembly, th e pro jec t i le is pressed into th e case neck with the neck free to expand radially to al low th e bead to ride over th e rear of th e pro jec t i le an d snap into the cr imp groove. At fi ring, with th e neck supported radially by th e chamber, th e bead is sheared ou t b j ' th e projecti le . force of 8 00 lb s i s required to shear ou t th e bead with th e neck supported radially. detai l drawing of th e plastic body, AAI Drawing No . 5 3 5 9 3 - 4 0 0 0 2 , is presented in Appendix I .

'Hui. ro sin No . L-1801

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Th e assembled munit ion contains 3 7 .3 grams of WC 8 7 0 double base propel lan t manufactured by Olin Mathieson. The total avai lable case volume is 2 . 4 4 cubic inches. The munit ion component weights are:

PLASTIC/ALUMINUM GAs

WC 8 7 0 PROPELLANT

M 5 5 A 2 PROJECTILE

TOTAL

38.0 rams

37 rams

9'>.0 rams

17 ^ . 3 grams

0.0838 b

(1.0822 b

1 1. 2 18 3 lb

(1.38/. 3 lb

Figure 2 rhewr. Lh

tnhlcd 20mm p! as( . 'a u m nuni artridge case

m u n i t i on.

b. PERFORMANCE

The charge weight of 3 7 .3 grams of WO 8 7 0 double base propellant loaded in th e 20mm plast ic rase pmv ided performance comparab le to th e M103 case which i s loaded to a charge weight of 39 to ( 0 gram;. All tests were with the M 5 5 A2 project i le . Based upon the results > f ten Mann Barrel fir ing tests of each, th e average of the velocit ies an d pressures which were recorded with each case are:

PLASTIC/ALUMINUM

M103 BRASS

VELOCITY

3355 fps

3350 fps

PEAK PRESSURE

5 2 , 8 0 0 ps i

51 , 000 psi

Presen ted in Figure 3 are typical examples of pressure .ersus t ime traces obtained from firing tests of th e plast ic /a luminum an d M103 cases. I t may be seen that th e pressure i n th e plast ic case rises at a sl ightly higher rate due to the smal ler case volume an d peaks approximately 0. 1 mil l isecond before that of th e M103 case. Th e pressure decavs are very similar. M6 1 operat ion was normal with th e plast ic case with proper feeding, chambering, and extract ion.

2 . 30MM CARTRIDGE CASE

a. PHYSICAL DESCRIPTION

The 30mm cart r idge case i s also assembled from tw o separate parts: an a luminum base and a plas t ic body, part numbers 5 3 5 9 3 - 4 0 0 11 and 5 3 5 9 3 - 4 0 0 0 5 , respectively. etai l prints of these par ts are given i n Appendix I I of this report. The base, machined from 7 0 7 5 -T6 , a high strength aluminum a l loy, has a tapered counterbore i th three bu t t r ess - shaped teeth forming a serrated

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Figure 2 . 20mm Plas t i c /Aluminum Cartridge Case Munit i on

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w O n

55640 - 1 41730-

1 7820- w 13910- 8 5

12 TIME MILLISECONDS

PLASTIC/ALUMINUM ARTRIDGE ASE W I T H 37.3 R A M S F C 70

D O U B L E B A S E ROPELLANT

12

TIME - MILLISECONDS

Ml03 RASS ARTRIDGE ASE WITH 9.5 R A M S F C 70

D O U B L E A S E ROPELLANT

Figure 3. Oscilloscope eadouts f ressure ersus im e Curve or 0mm artridge ases

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inside wal l as shewn in detai l " A" of 5 3 5 9 3 - 4 0 0 11 . Th e plastic body, injection molded f rom 4 3 percent glass - f i l led 61 2 nylon^.has at its lower en d a tapered outside wall with undercuts to match those of th e base.

The tapered section of th e plastic body has a nominal nterference of 0.005 inch with th e matching section of th e base. > ^ h e n the plastic body i s fo rced into th e base, i t snaps into place, forming a t ightly gripped joint. Th e plastic body has a wall thickness of 0 .0 9 6 inch an d an outside mean diameter of 1 . 8 7 6 inches which tapers from base en d to projectile en d at . 015 inch per inch. The diameter of th e projectile en d necks down i n inch of length to 1.301 nc h diameter i n order to accommodate th e T328 projectile. The inside wall of this lat ter mentioned diameter contains an 0 . 010

inch high x 0.062 inch wide bead which snaps into the crimping groove of th e T 3 2 8 project i le when the lat ter i s pressed into place as a final assembly operat ion.

The munit ion i s percussion fi red ut i l iz ing an experimental primer des ignated XM!15 which was developed by Frank ford Arsenal. Percussion f ir ing capabil it y wa s a contract requirement. The XM115 i s th e largest per- cuss ion- type primer available, an d i t was found > o perform sat is factor i ly without a booster charge at ambient temperature. At low temperatures ( -65 Ft, however, th e XM115 would no t ignite th e propellant charge. For lo w temperature tests, a Number 111 primer with a booster charge of 0. 3 gram of boron potass ium nitrate was used with satisfactory results. The assembled munit ion contains a propel lant charge of 160 grams of CI L 1379 C . The total nternal

case volume available

for propellant i s

1 1

cubic

inches.

Hie munit ion component weights are:

PLASTIC/ALUMINUMCASE

CIL 1379 C P RO P ELLA N T

T 3 2 8 P RO J ECTILE (WEIGHTED)

TOTAL

153.7 rams

160.0 rams

324.0 grams

0.3388 b

0.3527 b

0.71.43 lb

637.7 grams 1,4058 lb

Figures 4 nd 5 how he 30mm lastic/aluminum artridge case nd the assembled munition.

b. PERFORMANCE

Preliminary alculations showed that more than 150 grams of anadian Industries Limited piopellant CIL 1379 C ere required to ropel the projectile at muzzle elocity f 500 ps. Since the ehavior of the plastic/aluminum case as nknown nd espite nalysis indicating afe ssembly, the firsttwo artridges ere loaded with lower harges f 100 nd 150 rams in rder to scertain their eing afe. The remaining artridges contained 160 rams of ropellant.

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Figure 4 . 30mm Plas t i c /Aluminum Cartridge Case

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Figure 5 . 30mm Plast ic /Aluminum Cart r idge Case Munit ion

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The pro jec t i le used in th e assembly of these rounds was taken from 30mm ammunit ion, T P2 3 9 , supplied as GFP. The nose of th e project i le was removed, lead ;hot was added to th e body cavity until the weight of th e total project i le reached 32 4 grams.

At the required veloc i ty of 3500 fps, th e peak pressure i s approximate ly 4 2 , 0 0 0 psi. Figure 6 i s a typical pressure versus ti curve obtained from Mann barrel tests.

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T I M E

30MM PLASTIC/ALUMINUM CARTRIDGE CASE WITH IbO GRAMS OF CIL 1 3 7 9 C MUZZLE VELOCITY: 3 4 9 7 FPS

Figure 6 . Osci l loscope Readout of Pressure Versus Time Curve Fo r 30mm Plas t i c /Aluminum Cart r idge Case

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SECTION II I

TEST METHODS

1 . ELOCITY MEASUREMENT The projectile veloci ty was measured using counter chronographs i n con-

junction with l ight-sens i t ive veloci ty screens. Hie veloci ty screen consists of a tungsten lumi-1 ine tube light source winch shines di rec t ly across the project i le line of f l ight to a photoelectr ic tube. As th e project i le passes through th e screen, its shadow i s cast on th e photoelectr ic tube which then sends ou t an electr ical pulse. Th e electr ical pulse i s used to tr igger or to stop a counter chronograph. it h th e star t and stop screens se t at a known dis tance apart , th e recorded time to travel that dis tance will yield the average veloci ty between screens. For these tests, tw o sets of velocity record ing apparatus were used. The redundant set-up was arranged with th e start and stop screen pairs 10 feet apart , with th e first star' screen being 2 0 feet from th e muzz le and th e second start screen 34 feet from th e muzzle. The tw o readings were averaged .

2. RESSURE MEASUREMENT To obtain pressure versus time traces, a small hole was dr i l led i n the

case wall corresponding i n location to a port i n th e Mann barrel chamber. At f ir ing, th e gas pressure acts against a piston inserted i n the chamber port which, in turn, transmits th e force to a quartz dynamic force transducer. The t ransducer sends an electr ical pulse which i s then read-out on an osci l loscope and subsequent ly recorded on a Polaroid photograph. The result- ing photograph is a pressure versus time trace.

3 . TEM P ERATU RE EXTREME TESTS

The 30mm case was tested at - 65 F by packing th e outside of th e breech and chamber with dry ic e a f te r th e round wa s loaded into th e Mann barrel. A thermocouple attached to th e project i le nose monitored th e temperature. Upon reaching the des ired temperature, th e leads wero removed an d th e round was f ired. Figure 7 shows th e 30mm barrel .

The 20mm case was tested at temperature extremes by condit ioning th e loaded ammuni t ion separate ly f or a minimum of 4 hours at th e des ired temperature. or cold tests ( -6 5 F ) , a cold chamber operating wi th liquid CCL wa s ut i l ized . Fo r hot tests, an electr ical ly heated chamber was used. Both environmental chambers were equipped with automatic temperature controls. When ready for f ir ing, th e rounds were transported to th e test site in insulated containers , loaded in th e M61 and quickly f i red to minimize any temperature change. Elapsed time with th e rounds in th e container was about 2 minutes . ime to load th e rounds f rom the container into th e gun and f ire was about 1 0 seconds.

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SECTION IV

DEVELOPMENTAL TESTING

1 . 2 0 M M CARTRIDGE CASE

The basic concept for th e cartridge case design ha d btvn established previously during in-house projects conducted at AAI. The general des ign was scaled to 2 0 mm si^e using th e standard M 103 brass case as a model for th e outs ide dimens ions .

One of th e most important considerat ions at th e outset and throughout th e effo r t was the choice of materials for both case components . From past experience on in-house work, a luminum wa s a likely choice f or th e base. Since th e base i s unsupported at th e extractor groove at firing, high stress levels are experienced due to internal ga s pressure. high-s t rength alloy, 7075-T6 , was chosen for th e initial tests. This a l loy was eventua l ly incorporated into th e final des ign af ter t e s t ; ; with other aluminum al loys proved unsuccessful an d steel proved to be to o heavy for an y practical advantage.

In choosing a plastic for th e case body for the initial testing, a thorough material search was conducted with a study of th e propert ies of various types of plast ics to find those most compatible with the des ign

requirements of this feasibi l i ty study. Propert ies investigated included tensile strength, impact strength, elongation, water absorption, coeff ic ient of thermal expansion, f lammabil i ty, an d effect of various acids, alkalines, an d solvents . s a result, tw o materia l s stood ou t as having th e base combination of mechanical and chemical properties . They were polye thy lene an d nylon with various glass loadings for added strength. Specific formula- tions initially molded an d tested were:

POLYETHYLENE/ UNFILLED

DuPONT'S ALATHON)

POLYETHYLENE/307aGLASS

DuPONT1 S ALATHON)

66 NYLON/407, GLASS

LNP'S RF -1008)

61 2 NYLON/437, GLASS

DuPONT'S ZYTEL 77G 43)

The plas t ic case rounds were f i red in a Mann barrel with veloc i ty recorded. The general results were that th e 61 2 nylon/43 percent glass emerged as the best of these materials. It s combination of high impact and tensile strengths an d good elongation make it compatible with th e design requirements.

The glass-f i l l ed and unfi l led polye thy lene cases experienced circumferential fa i lures at th e joint area, an d the 66 n y l o n / 4 0 percent glass was too bri t t le to a l low assembly of th e project i le into th e case without crack ing th e neck.

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For th e test fi rings, standard 2 0 m m propeltant charges were loaded into plastic cases. he resul t ing veloci t ies were slightly higher than those of th e standard ammunition. They averaged 3 4 5 0 feet per second(approximately) while th e standard 2 0 r. a n del ivers 3380 fe per second. he increased veloci ty is at t r ibuted to a combinat ion of a s l igh t ly smaller case volume even though th e plastic case accepts th e full standard propel lan t charge and a diminished heat loss through th e plastic wal ls as compared to that of brass. The pressure in th e plastic case was also slightly higher than i n th e standard. Figure 8 is a comparison of th e pressure versus time traces obtained i n fi rings of a brass M103 and a plas t ic /a luminum case with ident ical charge loadings of 39 .5 grams of th e standard propellant WC870. he velocit ies an d peak pressures recorded for these tw o tests were:

BRASS M103 CASE 3367 fps 5 4 ,6 0 0 ps i

P LA S TIC/A LU M IN U M CASE 3401 fps 57 ,000 ps i

During these tests, projecti le retention was accompl ished by tw o methods: ( 1) a bead was molded into th e inside diameter of th e neck an d machined to the proper shape and this bead mated with a groove i n th e projecti le ( 2 ) th e projecti le was retained by adhesives. Problems were encountered with both methods. i th th e molded bead, some neck failures occurred. Upon firing, the case neck fa i l ed i n tension at th e bead locat ion indicating th e bead shear strength exceeded th e tensi le strength of th e case neck. To reduce th e bead shear strength, some areas of th e bead were removed making i t dis- continuous. This was unsuccessful as the forces were then local ised an d part ial neck failures occurred. Excellent results were obtained with a con- t inuous bead machined to hal f its original thickness. o neck fai lures occurred with this configuration.

A number of epoxies were tested for projecti le re tent ion charac te r ist i c swith actual test fi ring. The results were general ly inconsistent even when th e same type of epoxy is used repeatedly. uc h depends on the mix ratio, surface preparation, an d method of cure. he epoxies tested were:

A RM S TRO N G A - 3 1

RESIWELD 7004

BIGGS R - 3 9 3

SMOOTH-ON P B -1

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59028 t-i

* 4271 i

| 9514 to 00

* 4757 8 5

1 TIME - ILLISECONDS M103 RASS ARTRIDGE AS E WITH 39.5 RAMS F C870

DOUBLE BASE ROPELLANT

1

TIME - ILLISECONDS

PLASTIC/ALUMINUM ARTRIDGE AS E WITH 9.5 RAMS F C870

DOUBLE AS E ROPELLANT

Figure 8. Oscilloscope eadouts of ressure ersus Time urve or 0m m Cartridge ases

18

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f

Because of th e relatively Rood resul ts with the thinner, continuous machined bead and th e inconsistencies encountered with th e epoxies, i t wa s decided at this t ime to concentrate on per fec t ing a molded bead design f or pro jec t i le retent ion. The highest bul le t pull that can be ach ieved , 1000 lb , is governed by th e tensi le strength of th e case neck, th e size of which i s se t by projecti le outride diameter an d chamber inside diameter. The molded bead shear strength can, i f th e bead i s heavy enough, e ' reed the neck strength.

Since th e 61 2 n y lo n /4 3 percent glass gave th e best resul ts i n th e Mann barrel fi rings, a number of cases were molded for fi ring i n the M61 ^un.

In preparation f or fi ring in th e M61 cannon, the propel lant charge wa s downloaded for th e plas t ic ' a luminum case. Through a series of Mann barrel tests, a charge weight of 3 8 . 5 grams wa s established as that which del ivers th e same performance as th e standard round. This weight i s approximately a gram less than th e standard load.

Testing with th e M6 1 gu n wa s initiated with cycling tests to study th e integri ty of the plas t ic ' a luminum case. Assembled rounds were l inked and cycled through th e weapon without being fired. These t c . ^ t s , ' e i e successful , and preparations were made for fi ring tests. A single plast ic case round was at fi rs t f i red by manual ly loading i t into th e feeder. The round was fi red an d th e case wa s ejected proper ly. Two rounds with plas t ic 'a luminum cases wore then l inked at th e end of a belt of 1 2 standard rounds. Hie plas t ic/aluminum cases were placed last to insure that l i e gu n wou 'd be up to speed, thus subjecting them to the highest acceleration loads. These rounds were fired success fu l ly. The test wa s repeated with 6 plas t ic ' a luminum case rounds following 12 standard rounds; again th e f i ' ing wa s successful .

During these M61 tests, projecti le retent ion i n the case was acenrip! ished by a molded circumferent l ol bead in th e 'nside diameter of the neck which mates with th e crimp groove i n th e 20mm project i le. Fo r th e tests described above, th e bead was th e same size an d shape i n cross-sec t ion as Lhe projecti le crimp groove. As a result , some tension fai lures occurred i n th e case neck a t th e bead location indicat ing to o high a bead shear strength. Al l subse- quen t test ing was performed with th e continuous bead, bu t machined to hal f its original thickness, with excellent results. With this configuration, th e rear hal f of th e bead remains, contacting the rear of th e pro jec t i le

groove while th e rotating band bottoms on th e case mouth.

Test ing wa s resumed with th e modif ied retention bead in th e cases, an d no other neck fai lures have occurred. The final M6 1 test of this initial series involved th e f ir ing of a belt of 2 4 plas t ic /a luminum case rounds with th e test recorded by both high speed and conventional movie cameras. l l rounds fi red, an d al l cases were extracted an d ejected properly.

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.02 04 .06 .03 .10 .12 TIME - SECONDS . 1 4 . 16 Figure 9 . Osci l loscope Readout of Timing Trace for M6 1

Cannon Firing at 4300 Rounds Per Minute

2 1

f>7-


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0.030

SS S S

^L .047

,J*J

'v S S 5 '.

BEFORE MODIFICATION

F 0.065

S SST ~JL 0.070

r- T -S.

s s s c \ s^ szSzs

REINFORCED WALL

Figure 10. ase Wall Reinforc ing to Resist Link Squeeze

Scale: 2/1

22

"J"~ f r"r " "It*"?? f" 1 - i. i ^ fr .


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Mold C.;vily

Core

Diaj.luayin ate

Figure 11. Mold Gating t Start of Development

Scale: 2/1

2 4

Ty~-


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M t I d Cavi ty

Cure

Larger Gating Cavi t ies

Figure 12 . mproved Mold Gating

Scale: /1

25

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C a s e s of 61 2 n y l o n / 4 3 p e r c e n t g l a s s w h e n t e s t e d h o t y i e l d e d e x c e l l e n t r e s u l t s . G u n o p e r a t i o n wa s n o r m a l , a n d th e c a s e s w e r e n o t d a m a g e d a t f i r i n g a nd e j e c t i o n . W h e n t e s t f i r e d c o l d , h o w e v e r, some p r o b l e m s w e r e e x p e r i e n c e d . A p p r o x i m a t e l y 3 0 p e r c e n t of th e c o l d f i r i n g s w e r e s u c c e s s f u l , b u t in th e m a j o r i t y of c a s e s f i r e d , l o n g i t u d i n a l c r a c k s o c c u r r e d n th e p l a s t i c p a r t of th e c a s e . Th e c r a c k s u s u a l l y r an th e e n t i r e l e n g t h of th e p l a s t i c b o d y, bu t th e c a s e i t s e l f r e m a i n e d in tac t as th e p l a s t i c s h o w e d th e a b i l i t y t o r e s e a l i t s e l f . T h e r e wa s e v i d e n c e of gas l e a k a g e n some of th e c a s e s a t th e c r a c k s , b u t th e l e a k a g e a p p e a r e d t o b e m i n i m a l d u e to th e p l a s t i c h e a t i n g a t th e c r a c k e d a r e a , f u s i n g and r e s e a l i n g . A l t h o u g h t he cracks in th e p l a s t i c e x t e n d e d in to t h e joint a r e a s u r r o u n d e d b y the a l u m i n u m , n m o s t t e s t s t h e a l u m i n u m wa s u n a f f e c t e d . F o r al l e n v i r o n m e n t a l t e s t s , g u n

o p e r a t i o n wa s n o r m a l .

S o m e t e s t s were c o n d u c t e d n w h i c h th e g l a s s c o n t e n t in th e n y l o n wa s r e d u c e d f r o m 4 3 p e r c e n t to 2 5 p e r c e n t in a n e f f o r t to i n c r e a s e th e e l o n g a t i o n p r o p e r t i e s . Cracks s t i l l o c c u r r e d , h o w e v e r. W h i l e t he elongation propert ies h a d b e e n i m p r o v e d by d e c r e a s i n g th e g l a s s c o n t e n t , the i m p a c t s t r e n g t h p r o p e r t i e s h a d b e e n d e g r a d e d and i m p a c t s t rength . s a prime considera t ion f o r p r o p e r c o l d t e m p e r a t u r e o p e r a t i o n ,

Some t e s t i n g a t t e m p e r a t u r e extremes were conducted with cases of polycarbonate (G . E . ' s Lexan 1 91 ) . The polycarbonate did no t crack at - 6 5 " F, but n e c k s e p a r a t i o n s a t the r e t e n t i o n bead occurred, Polycarbonate i s notch s e n s i t i v e , a n d th e m a c h i n i n g r e q u i r e d a t the b e a d to achieve th e proper bead

t h i c k n e s s h a d c r e a t e d a s t r e s s c o n c e n t r a t i o n point. This, in conjunction wi t h th e l o w e r t e n s i l e s t r e n g t h o f th e unfi l led mater ia l , had resulted i n n e c k t e n s i l e f a i l u r e s . W h e n th e p o l y c a r b o n a t e cases were tested at 160F, t h e y e x p e r i e n c e d s e p a r a t i o n f a i l u r e s a t th e plas t ic /a luminum joint during d e l i n k i n g p r i o r to c h a m b e r i n g . Wi t h th e M1 4 link hold ing th e case tore an d a f t , th e f e e d e r in the M61 g u n s t r i p s th e r o u n d out of th e link with a ramp t h a t a p p l i e s a side f o r c e o r . th e m i d d l e of t he case. This stresses the joint a r e a of th e c a s e , p a r t i c u l a r l y th e plastic material. Hi e lower tensile s t r e s s o f th e u n f i l l e d m a t e r i a l c o u l d not wi t h s t and t he s i d e loads. Once d e l i n k e d , th e p o l y c a r b o n a t e c a s e s c o u l d be c h a m b e r e d , fired, and extracted n o r m a l l y in 160F.

It wa s e l t , t h a t th e m e c h a n i c a l p r o b l e m s w i t h t he p o l y c a r b o n a t e c a s e s

c o u l d b e o v e r c o m e wi th m i n o r c a s e c o n f i g u r a t i o n c h a n g e d a nd tha t a w o r k a b l e c a s e c o u l u b e d e v e l o p e d . On e p r o b l e m d o e s e x i s t in u s i n g p o l y c a r b o n a t e m a t e r i a l : i t s s u s c e p t i b i l i t y to s t r e s s c raz i ng a f t e r e x p o s u r e t o c h l o r i n a t e d h y d r o c a r b o n s . H i w e v e r, p o l y c a r b o n a t e r e m a i n e d i n c o n s i d e r a t i o n as a b a c k - u p c a n d i d a t e m a t e r i a l f or th e c a s e in t h e e v e n t t h a t th e c o l d c r a c k i n g o f t h e n y l o n c o u l d n ot b e e l i m i n a t e d .

A t t h i s po in t , c o l d t e m p e r a t u r e o p e r a t i o n h a d b e e n e s t a b l i s h e d as th e m o s t s e v e r e o p e r a t i n g e n v i r o n m e n t f or th e p l a s t i c m a t e r i a l . Th e 61 2 n y l o n /

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Figure 1 3 . Typical Case Fail ures at Low Tempera tures

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General Electric General Electr ic Dow Corning Biggs Biggs Thiokol

RTV 1 5 4 ray RTV 1 03 ed RTV 7 33 R - 3 1 3 R - 3 9 3 LP-2

(Sil icone Rubber) (Sil icone Rubber) (Sil icone Rubber) (Epoxy) (Epoxy) (Polysulf ide Rubber)

Each sealant was tested f or seal ing proper t ies at both the case joint and th e n eck - i n -wa t e r immers ion tests. eneral ly stated, th e tes t ing was successful as all sealants e ff ec ted water t ight seals at th e joint and neck. In addition, on e case was assembled with no sealant at th e case joint, and i t was found to be per fec t ly watertight.

The first por t ion of th e test ing involved immersion of th e sealed area of par t ia l ly assembled rounds to check visual ly for leakage. or this, 7 cases were assembled, 6 with sealants at th e case joints and staking lacquer at th e pr imers and with no sealant or staking lacquer. Th e cases were placed in water, and they f loated base downward with th e top of th e base approximate ly inch below the surface. They were left in th e water for 72 hours with th e result that al l case joints were water t ight including th e on e with no sealant.

The sealants were tested at th e neck of th e case by assembling a projectile into only th e plastic body of th e case with sealant and immersing th e assembly nose downward with th e neck 2 - 1 / 2 inches below th e surface. The

samples

were l ef t for

72-hour s and again

al l

seals

were

watertight.

Th e second por t ion of th e testing cons is ted of immersion of assembled rounds in water and later f ir ing th e rounds. Fo r this, th e same cases that had been used in th e case joint sealant tests were util ized. They were loaded and assembled wi th sealant a t th e case neck/project i le interface and immersed in wat e r to a dep th of 4 inches for 4 8 hours. In this instance only RTV's were used at th e neck due to their easy application. able II lists the types of sealants used and their point of application.

TABLE II . TYPES OF SEALANTS A ND POINTS OF APPLICATION

Round Sealant

Case Case Staking Lacquer

No. Jo in t Neck Primer

1 RTV 1 5 4 RT V 1 5 4 Yes 2 RTV 7 33 RTV 733 Yes 3 RTV 106 RT V 106 Yes 4 R - 3 1 3 RTV 733 Yes 5 R - 3 9 3 RTV 1 06 Yes 6 LP-2 RTV 7 33 Yes 7 None RTV 1 5 4 No

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A f te r th e soak, th e rounds were f i red in a Mann barrel. eloci ty measure- ments indicated that th e performance was normal for al l th e rounds.

From these tests i t i s concluded that th e sealing of th e case joint an d neck will present no par t icular problem. Because of it s conf igurat ion th e case joint i s easily adaptable to sealants and i s , i n fact, a natural seal.

Concurrent with investigations into th e use of sealants, work ha d continued on th e cold cracking problem with th e test ing of addit ional materials. They were LNP's 61 2 N ylon /45 percent glass an d Fiberf i l ' s 61 0 N ylon /40 percent glass. ei ther was successful .

The use of a new plastic material was investigated for possible app l ica- tion i n the 20mm plas t ic /a luminum case design. Th e new thermoplastic material, Polyarylsulfone, produced by th e 3M Company, i s a temperature- res is tant mater ia l and exhibits good properties for this appl icat ion especial ly i n th e low temperature ( -65F) area. Hits material, however, was found to require molding pressures in th e 40 , 000 to 50 ,000 ps i range, which i s no t achieveable in AAl ' s plastic molding faci l i ty. Arrangements would have to be made to test mold th e material at a subcontractor with facil i t ies tn mold at th e high pressures. In addit ion, th e us e of this material would also require extensive modif icat ion to th e existing case mold. The mold would have to be heated to 4 5 0 F electr ical ly instead of using th e exis t ing water heat ing methods. Heating elements, controls, and associated hardware necessary for th o modif i - cation were investigated and selected for possible procurement. \ serious drawback to th e us e of this material was its price of approximately $2 5 lb in small quanti t ies. In larger quanti t ies, th e projected price was still prohibit ive.

With al l the factors taken into account, it was ut l imately decided that th e molding of th e case from Polyarylsulfone would no t be done. I I was reasoned that th e high raw material cost would place th e plas t ic /a luminum case in an unfavorable price situation. In addition, th e extensive mold modif ications required to mold this material would ruin th e mold for other less expens ive materials.

At this point , th e original contract en d date ha d been reached. Basic feas ib i l i ty of th e 20mm plas t ic /a luminum car t r idge case ha d been proven with successful operation of the case in the M6 1 at ambient temperatures. Extra work had been performed by test ing t . temperature extremes an d cold f ir ing ( -65 F ) ha d been isolated as th e most severe operat ing environment a ff ec t ing plas t ic /a luminum cases. Th e fa i lu re rate at cold temperatures had been reduced f rom 60 percent dur ing th e initial tests to 4 0 percent with th e standard W C 8 7 0 charge and to 15 percent with lower charge weights. A total of 5 68 plas t ic /a luminum case rounds ha d been tested, 1 2 3 in th e Mann barrel an d 4 4 5 in th e M61. The best material for th e plastic body wa s 61 2 nylon/ 43 percen t glass (DuPont 's Zytel 77 G43 ) and 7 0 7 5 - T 6 aluminum for th e base.

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p

The cold crack ing that ha d occurred did no t appear to affect per fo rmance or gun operation in most instances. The plast ic tended to refuze at th e cracked area. i th approx imate ly 5 percent of those that cracked, however, more severe erosion of th e hase occurred result ing i n a base burn th iugh at th e extractor groove. ro m visual inspect ion of the fai lures, i t appeared that the aluminum base had cracked longitudinally on th e side a l lowing th e propel lan t gases to escape an d subsequently erode th e base. Figure 1 4 shows base erosion failure.

One prob lem area noted dur ing testing and the possible source of case fa i lures i s the slanted bolt face of each bolt in th e M61 gun. Th e bolt face ha s a slant of 0 - 4 5 ' f rom th e plane normal to th e axis of th e barrel .

At fi ring, th e base deforms to contact th e full c

ace. Th e def lec t ion introduces added stresses into th e base as well as the plast ic body. At ambient an d high temperatures, this i s of no consequence bu t at low temperatures, i t can contribute to fai lures since at lo w temperatures, the material properties of e longat ion and impact strength are degraded.

START OF WORK ON EXTENDED CONTRACT

With th e receipt of addit ional funding and an extension of th e contract en d date, work wa s resumed i n th e development of th e 20mm plas t ic ' a luminum car t r idge case. The goneral des ign of th e base /case joint was invest igated an d modified. To facil i tate manufactur ing operations, the number of buttresses in the joint was reduced to tw o with th e elimination of th e forward tooth.

A par t icu lar ly important change to th e a luminum base wa s o n - - of increas- in g the outside diameter to produce a closer f it with th e chambt-r. Pre- viously, a c learance of 0.004 to 0 .0 1 0 inch on th e diameter had existed between base an d chamber. At f i r ing ,wi th th e base se t back against th e slanted bolt face, the c learance a l lowed th e base to cock slightly, imposing added stresses on th e plastic body at th e joint, possibly initiating th e cold cricking. In addit ion, th e base had to expand at f ir ing by th e amount of th e c learance to contact th e chamber. t -6 5 F under shock loading, this may have been an excessive amount of expansion. It is theorized that occasiona l ly ( ? percent of th e fi rings) th e base did crack due to th e cocking and excess ive expans ion,and a subsequent ga s erosion an d burn- th rough at th e extractor groove resulted. The base diameter was increased to achieve a f it with th e chamber ranging f rom a 0.001 inch press to a 0.003 inch clearance on th e diameter. This minimized th e cock ing of th e base with al l deformation due to th e slanted bolt face occurr ing in the rear of th e base. In this way, no addit ional loads were imposed on the plastic body.

It is important to note that this modif icat ion completely eliminated base burn- th roughs an d s ign i f ican t ly reduced the incidence of cold cracking of th e plastic body f rom 4 0 percent to approx imate ly 10 percent .

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Figure 1 4 . Base Erosion Fai lure on 2 0 mm Plas t ic /Al uminum Case

34

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With th e star t of work on th e modif ied contract , th e 20mm mold was improved. Th e changes to th e moid we' ade i n three main areas. Th e first involved th e fabrication of a new cor - . d gate suggested by tho AFATL project engineer to achieve bet ter glass dispers ion within the molded part. The second change was th e eliminati of the forward base locking buttress which was judged to be unnecessary f or th e retention of th e cartridge base. Also, th e f irst buttress had been a source of an occasional small circumferential crack in th e aluminum base. Thus, th e case/base joint des ign no w has 2 locking buttresses instead of 3 . inally, th e third modif icat ion involved th e molding in place of th e projectile retention bead, suggested by AFATL project engineer, with th e proper configuration. This el iminates th e bead machin ing operation so that th e only machin ing required af te r molding is to cut th e case to th e proper length. i th th e molding of the retention b^ad to th e proper size, th e problem of neck separations at low temperatures was eliminated. Figures 15 and 16 i l lustrate th e modif icat ions .

With th e resumption of testing, a number of new mater ia ls were molded in addi t ion to th e 61 2 ny lon /43 percent glass. They were:

ASTMAN'STENITE POLYTEREPHTHALATE (PTMT) UNFILLED

ASTMAN'S TENITE POLYTERE PUT" IA LATE (PTMT)/20 PERCENT LASS

ULS'S TYPE 12 NYLON/30 PERCENT LASS

ELANESE'S 917 THERMOPLASTIC ESIN/30 PERCENT LASS

The results hen fired t -65 ere:

TMT/UNFILLED (6 ROUNDS) - ALL CASES CRACKED T IRING

0 PTMT/20 ERCENT LASS (6 ROUNDS)- LL CASES CRACKED T IRING

YPE 12 NYLON/30 PERCENT USS (6 ROirNDS)

With this material, 5 f the ases ere roken in the feeder uring delinking. This appeared to e ue to igh friction etween the case nd link hich caused excessive forces at elinking. The 1 round that passed

through the feeder intact as fired ith o ase amage.

9 1 7 / 3 0 P ERCEN T GLASS Cases could no t be assembled without cracking the case necks, which

indicated extreme brittleness.

It should be noted that in using mater ia ls that are no t glass f i l led, such as PTMT, th e resul t ing molded case is cons iderably undersize. The under- slze condit ion may have contr ibuted to th e case failures. problem exists

35

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Mold Cavi ty

Core

Molded Retent ion Bead

Gate

Figure 15 . old Changes at Start of Modi f i ed Contrac t

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rv

%^s~ // /'. - '

_ .c

;p=p=}

Three Buttresses

ra

Two Buttresses

Figure 16. ocking Buttress Modi f i ca t ion

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in that th e same mold which has been designed to r th e lower shrinkages of glass f i l led mater ia ls is also used in mo ld ing unf i l led plastics with their higher shrinkages. I t becomes diff icu l t to fair ly evaluate th e unf i l led materials. It is fel t , however, that in this ins tance ,because of the severity of th e cracks, th e unf i l led P TM T would no t per fo rm satisfactorily regardless of th e case size.

In addi t ion to th e regu lar cartridge base mater ia l , 7 0 7 5 - T 6 aluminum al loy, other mater ia ls were tested an d evaluated. They were 2 0 2 4 - T 4 an d 6 0 6 1 - T 6 aluminum alloys and steel. All bases fabricated hencefor th re f lec ted the increased outside diameter to achieve a better chamber f it discussed earl ier.

2 0 2 4 - T 4 BASE S

General results are that the slightly larger diameter base which provides a close chamber f it i s th e bet ter conf igurat ion since i t prevents cocking in th e hamber due to th e slanted bolt face. Also, it was fel t that th e us e of the softer 2 0 2 4 - T 4 aluminum instead of th e 7 0 7 5 - T 6 might have merit since it will defo rm easier, at fi ring, on th e slanted bolt face and no t transmit as high a force to th e plastic portion of th e case. Testing at - 65 F with 2 0 2 4 - T 4 did provide good results, bu t at 160F he material was severely deformed with some small circumferential spl i ts occurring. The material di d no t have th e tensile strength to withs tand th e high stress levels when fi red at e leva ted temperatures.

6 0 6 1 - T 6 RASES

Cases with bases of 6 0 6 1 - T 6 were test fi red at ambient temperatures an d -65F. The resul ts at ambient were sa t i s fac to ry bu t those at -65 F were not. Although 30 percen t functioned properly, th e remainder cracked at f ir ing or were broken in th e feeder at delinking. Hi e feed ing problem appeared to be caused by high fr ict ion between th e 6061 base an d link which transmitted excess ive stresses to th e plastic at delinking.

The other standard a luminum a l loys are lower in tensile strength an d therefo re were no t tested. Based on these tests, 7 0 7 5 - T 6 appears to be th e best of th e various aluminum al loy grades f or appl icat ion in this area.

ST E E L BASES

The us e of steel bases was thoroughly investigated and found to be feasible. The pr imary advantage with steel i s that it wil l no t burn under th e inf luence of high pressures an d temperatures as aluminum does so that there is no poss ib i l i ty of a burn- th rough .

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V

Cases with steel bases were fi red in th e Mann barrel and M61 to determine th e best configuration for th e steel base. Steel bases identical to th e aluminum base configuration were f i red success ful ly at ambien t temperature. Effor t was made to make the base as l ight as possible by designing thinner walls and ut i l iz ing th e proper heat treatment. It was at tempted to deve lop a steel base that would ul t imately compare favorab ly with th e aluminum base in both weight and performance. Heat treated 4 3 4 0 steel bases were designed to reduce th e weight as much as possible yet till withs tand th e f ir ing pressures, Twenty-one f ir ings were conducted in which various combinations of base thickness, height, and heat treatment were t r ied to produce a minimum weight case. The result ing plastic/steel case which was fi red success ful ly in th e M61 af ter being tested in th e Mann barrel weighed 4 6 grams as compared to 34 grams

f or th e plas t ic /a luminum case and 1 2 0 grans f or th e Ml03 brass case.

With th e apparent elimination of th e aluminum base burn- throughs an d con- sideration of the weight increase using steel bases, it was ul t imately decided that 7 0 7 5 - T 6 aluminum would be th e netter choice of mater ia ls for th e cartridge base. With th e establishment of th e final conf igurat ion an d material for th e base, al l effo r t was then concentrated on th e prob lem of cold cracking of the plastic body.

Test ing continued with th e evaluat ion of ne w mater ia ls an d improved versions of those a l ready tried as these became available.

POLYCARBONATE/4 PERCENT AND 8 PERCENT GLASS (G .E . ' S LEXAN 141)

Prior tests ha d been made with unf i l led an d 2 0 percent glass - re inforced Lexan. The results of these tests were that th e unf i l led material worked at - 6 5 F bu t was no t strong enough at +160F, an d the glass - f i l led material was to o brittle even under ambient conditions. Tests were conducted at - 65 F with 4 percent an d 8 percent f i l l ed material in an effort to strengthen th e unf i l led mater ia l without maKing i t to o brit t le . Both mater ia ls were successfu l in a l imited number of tests at th e cold conditions. High temperature f ir ings, however, were unsuccessful as tensile failures occurred.

P O LY CA RBO N ATE/10 PERCENT GLASS ( G . E . ' S LEXAN 500)

Lexan 5 0 0 /1 0 percent glass showed some promise as 12 rounds were f i red at

- 6 5 F with no failures of any kind. Si x were then

f ired ho t at 160F,

an d 5 of the 6 cases separated at th e joint probably because of high fr ict ion between th e Lexan an d chamber an d lower material strength at th e joint area. Test ing with polycarbonate was discontinued.

P O LY ES TER/30 PERCENT GLASS (G .E . ' S VALOX 420 )

The thermoplastic polyester, G.E . ' s Valox 4 2 0 * ' ich contains 30 percent glass, was molded and test f ired in th e M61 at - 65 . . he results were no t sat is factory as al l cases fai led. alox 4 2 0 apparent ly i s no t suited for th e high stresses encountered in case operation.

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ETFE FLUOROPOLYMER/UNFILLED AND 2 5 PERCENT GLASS (DUPONT'S TEFZEL)

Cases of f i l led an d unf i l led Tefze l were molded for test ing with DuPont technical personnel present to . * > r1st in es tabl ish ing proper molding para- meters. One immedia te ly apparent problem was th e large amount of shrinkage in th e molded part result ing in unders ized cases.

Firing of th e cases at - 65 F yie lded a failure rate of !5 percent m - . ' i n l y due to th e cases being undersize. It was felt that, with a mold ta Holed to th e requirements of Tefzel , a satisfactory case could be produced. Th e price of Tefze l , however, appears to be prohibit ive. Present ly th e cost i s $9 per pound. an d th e projected price in quantity wil! be $ 6 . 5 0 per pound at th e very minimum. This will no t be compet i t ive whan c o . , . p a r e d to the current price of DuPont 61 2 ny lon /50 percent glass, which i s $2.00. N ' o further work was performed with Tefzel .

61 2 NYLON ( M ODI FI E D) /4 3 PERCENT GUSS (DUPONT'S FE 5 024 )

The special formulation of glass - f i l led nylon with added tougheners des ignated FE 50 2 4 was formulated by DuPont, molded , an d test fi red at - 65 F . The results of th e tests were unsat is factory in that th e failure- rate wa s 67 percent. DuPont was not i f ied of this and agreed to tr y another approach toward a solution, which has to increase the glass content of th e material . Since considera l e improvement i n th e failure rate was ach ieved as addit ional glass was addec up to 4 3 percent, th e maximum available at that Lime, i t was agreed to tr y percent glass. This material was then formulated by DuPont.

61 2 NYLON/50 PERCENT GLASS (DUPONT*S FE 5030)

The initial tests with 61 2 ny lon /50 percent glass were highly successful. Thirty f ir ings were conducted, 20 at -65F, an d 10 at 1 6 0 ' F with no failures of any kind. This was th e best performance to date of any material .

TYPE 1 2 NYLON/30 PERCENT GLASS LONG FIBERS (HLS' S )

Hls ha d prepared a type 12 nylon with 0 .2 5 inch long f ibers instead of their standard 0 . 1 2 5 inch fibers . The initial f ir ings at - 6 5 F with this mater ia l were moderate ly successful in that, of 2 5 fi rings, only 2 failures occurred. hen f i red hot, however, tensile failures occurred at th e transi-

tion point between th e heavier, reinforced section and th e thinner wall aft. The tensile failures were attr ibuted to th e lower tensile strength of the mater ia l at e leva ted temperatures. The type 1 2 nylon did show promise with th e success ful cold tests, and th e addition of more glass would help performance, both hot and cold.

At this t ime, tw o mater ia ls emerged as the prime candidates for app l ica- tion in th e 20mm plas t ic /a luminum case design. They were 61 2 ny lon /50 percent glass an d type 12 nylon with a slight edge given to th e 61 2 nylon because of its higher glass content.

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It was decided to test larger quant i t ies of rounds to bet ter es tabl ish th e integrity of cases using 61 2 n y l o n / 5 0 percent glass. The initial tests cons is ted of 2 0 f ir ings in th Mann barrel under th e ambient conditions to compare performance with th e standard brass ammunit ion . The charge wa s es tabl ished at 3 7 .5 grams of W C 8 7 0 propel lant which i s 0. 2 gram more than previous ly used on th e successful firings. Compar ison of th e performance of th e tw o rounds i s :

M103

PLASTIC/ ALUM INUM

AVERAGE EA K AVERAGE PRESSURE VELOCITY

50,765 3349

48.138 3

3346

Using this propel lant charge, 5 4 cases were fi red i n the M6 1 a f te r being condit ioned at -65F for four hours. Eleven cases ,or approximate ly 2 0 percent, cracked. n immediate investigation was conducted to determine th e reason Cor these failures since previous tests of this material had produced 2 0 fir ings with nn failures occurr ing under th e cold conditions. To determine th e i m p a c t : of th e increased charge, twelve cases were f ired with 3 7 .3 grams of propel lant at - 6 5 F and four cracked. This indicated that th e increased charge was no t th e cause of th e failures. I t wa s then obvious that th e increased failure rite wa s due to loss of proper t ies in th e plastic component, an d DuPont wa s contacted tc evaluate th e situation. Two poss ib le probier; areas were recommended for investigation, both of which could cause a loss of proper t ies in th e molded parts: (1) DuPont suggested raising th e melt temperature to 560F instead of 510F as ha d previous ly been used. New cases were molded, an d twelve were f ired at -65F with only on e fa i lure which i s a s ignif icant improvement; ( 2 ) DuPont also suggested that excess ive moisture in th e plastic ra w mater ia l prior to molding could have caused a loss in th e phys ical properties . Even though th e material was dr ied prior to molding, th e moisture content could still be greater than as supplied. DuPont then suppl ied a new lo t of material to determine th e effect of th e moisture content.

Addit ional tests were conducted with the ne w batch of DuPont ' s 5 0 percent glass-f i l l ed 61 2 nylon which wa s molded immediate ly af ter opening the sealed container. The mois ture content was checked and found to be . 024 percent, an except ional ly low amount. These cases were molded at th e higher melt temperature. Fif ty cases were f ired at - 65 F an d 8 cracked.

Theoriz ing that high case f r ic t ion might cause excess ive stresses in th e plas t ic body, some cases were Teflon-coated to evaluate this concept. Th e 61 2 n y l o n / 5 0 percent glass cases were sprayed with DuPont ' s Tef lon Resin No. 8 5 0 - 2 0 4 , a ir dried, and then f l ame- fuzed pr ior to assembly. It was ant ic ipa ted that th e Tef lon coating would provide decreased frict ion wh en del inking,

3 Due o subsequent mold hanges which educed he nternal ase olume, he peak pressure n he final ase onfiguration

53.000 pn s tated earlier

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feeding, an d extract ing and would a lso provide a sealed external surface on th e nylon. Upon tes t ing th e coated cases at - 6 5 F, the failure rate had been reduced to 7 percent bu t th e cracks had no t been el iminated .

Another type of coating on th e 61 2 n y l o n / 5 0 percent glass case was tried. Xylon, made by Whit ford Corporation, has bet ter adhesion and lower curing temperature than does Teflon. Testing, however, yielded poor results with a fa i lure rate of 30 percent. Some of th e fa i lures were tensile failures indicating an increr.se in fr ict ion.

The us e of coatings on th e nylon case was general ly undes irable . Tef lon does no t adhere wel l since th e proper curing temperature cannot be achieved

without adverse ly a ffec t i n g th e nylon. The Xylon coat ing increased th e coeff ic ien t of f r ic t ion rather than decreas ing i t . The molded nylon case normally ha s a low coeff ic ient of friction (on th e order of . 1 ) and i s d i f f i cu l t to improve upon Any further evaluat ions of coatings for th e plas t ic body were discont inued.

MOLD CHANGES

A t th e request of th e government program manager, a mold change wa s made to th e core to eliminate < " h e sharp transition between th e thick an d thin sections of th e case at th e forward, re inforced area, This area could con- tain residual stresses result ing at molding and cool ing at d i f f e r en t rates because of th e d i f f e r ences i n thickness. The thin section was def in i te ly a weak point in tension as evidenced by th e tensile failures at high temperatures with the type 1 2 nylon/30 percent glass cases. Figure 1 7 i l lustrates th e change.

An improved formulat ion of type 12 nylon with increased glass loading up to 5 0 percent was received at this time. The type 1 2 nylon/30 percent glass had achieved moderate success at low temperature, bu t tensile failures occurred at high temperatures. The 50 percent glass loading would make i t competi t ive with the 61 2 n y l o n / 5 0 percent glass.

With th e core modif ica t ion completed, cases of 61 2 nylon/50 percent glass and type 1 2 n y l o n / 5 0 percent glass were fabricated for testing. A thorough dimensional inspection of th e assembled cases were conducted prior to fir ing

o at - 65 F to determine if those that fai l are dimensionaIly di fferen t from th e others. Figure 16 i s an i l lus t ra t ion of the per t inent dimens ions that were checked.

Firing of th e cases at - 65 F produced failure rates of 33 percent for th e 61 2 nylon an d 8 percent for th e type 1 2 nylon. The inspection record was then compared with th e test results. In general, th e results indicated that th e cracks occurred when th e "B" dimension ( re fer to Figure 1 8 ) was on

the lo w side of th e average. Based on this reasoning, th e mold was again

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ff

irl

Case WJLI Before Modifikation

Figure 7. 2 0 m m ase Wall Modification

Scale: 2/1

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n i

\1 . 000

i

H " f_i

0 . 9 4 2 Datum Di a

Figure 18 . Dimensions Inspected on 2 0 mm Plast ic /Aluminum Case

Scale: 1/1

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*

modi f i ed to increase this dimension. The increase amounted to 0 .0045 inch on th e diameter. This achieves a l ine- to - l ine contact with th e minimum M61 chamber size at ambient temperatures.

Cases of 61 2 ny lon /50 percent glass were molded in th e a l te red mold, inspected, and f ired at -65F. ut of 2 4 fi red, ^ cracked and 1 separation occurred, th e f irst f or this material . The failure rate had been reduced by th e increased case size. The reason for th e separation was no t determined, bu t a t ight f it be tw een th e case and base was suspected. Presently, there i s a 0 .0 1 5 inch squeeze on the diameter by th e aluminum base at th e plastic buttress area. Examinat ion of th e cracked cases indicated that this stressed area may be th e source of th e cracks at f i r ing and may have caused th e case

separation. To ver ify this, a number of bases were modif ied to rel ieve th e squeeze on th e plastic at assembly. The f it was changed to l ine- to - l ine to a 0.002 press. Of 2 4 of these fi red cold, cracked and 2 separated, indicating that the squeeze on th e plastic apparent ly ha d no t been th e cause of th e cold fai lures.

Cases of type 12 ny lon /50 percent glass were then molded for testing. Thase cases ref lected th e latest mold changes of increased outside diameter an d heav ier wall . Test results were highly successful t . that no failures of any kind occurred at high or low temperatures. With successful initial tests at -65F, f ir ings were continued unti l a total of 100 rounds ha d been fi red at -65F, with no case fai lures. A total of 19 fi rings were made at 160F which were al l successful .

This is th e best per fo rmance of any plastic mater ia l tested dur ing th e course of this contract , an d i t was incorporated into th e final des ign specif ications f or th e case.

During the course of this contract , a total of 1301 plastic case 20mm rounds ha s been test i red in th e M61 an d in a Mann barrel . The total includes those f i red at high an d low temperature extremes. The breakdown i s as fol lows:

MANN BARREL 1 4 6

M61 ( - 6 5 F ) 8 4 4

M61 (AMEIENT) 2 1 5

M61 (+160F) - 9 6

TOTAL 1301

Appendix I I I contains drawings of th e 20mm plastic case mold. ppend ix VI contains a record of test f ir ings as related to mold changes.

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During th e course of development , 2 8 types of plastic mater ia ls were tested. They ar e l is ted in Table III.

TA BLE III . LISTING OF TYPES OF PLASTIC M ATERIA LS TESTED

PLASTIC PERCENTAGE OF FIBERGLASS BY WEIGHT

+ MANUFACTURER, S P ECIF ICATIO N

ETFE Fluoropolymer Unfi l led DuPont (Tefzel 200)

ETFE Fluoropolymer 2 0 DuPont (Tefzel 200M

66 Nylon 4 0 LNP, ( RF- 1 0 0 8 )

61 0 Nylon 4 0 LNP, ( QF- 1 0 0 8 )

61 0 Nylon 4 0 Fiberf i l ( G 1 2 / 4 0 Nylafil)

61 0 Nylon 50 LNP, ( QF- 1 0 0 - 1 0 )

61 2 Nylon Unfi l led DuPont (Zytel 1 5 1 )

61 2 Nylon 25 DuPont (Special Mix)

61 2 Nylon 3 3 DuPont (Zytel 77G33)

(612 Nylon ( 7 7 %) ) 1 Nylon ( 2 3 7 . ) f

3 3 DuPontZytel 77G4 3 I w 1 Mi v IZytel 2 11

61 2 Nylon 43 DuPont (Zytel 7 7 G4 3 )

61 2 Nylon modif ied 43 DuPont ( FE 50 24 )

61 2 Nylon 45 LN P ( IS -100 9 ) 61 2 Nylon 5 0 DuPont ( FE 5030)

Type 1 2 Nylon 30 Hls ( L 1y30)

Type 1 2 Nylon * 50 Hls (Thermofil N9 - 5 0 0 0 - FG)

Polycarbonate Unfi l led G.E. (Lexan 141)



Polycarbonate 4 G.E. (Lexan 141)

Polycarbonate 8 G.E. (Lexan lAl)

Polycarbonate 10 G.E. Lexan 500)

Polycarbonate 4 0 G.E. (Lexan 500)

P o lyes te r Unfi l led Eas tman (PTMT 6PRO)

Polyes ter 2 0 Eas tman (PTMT 6G91)

P o lyes te r 30 G.E. ( Va l . o x 42 0 )

Polyethylene Unfi l led DuPont (Alathon)

P o lye thy lene 30 DuPont (Alathon)

* se d on f inal + PP Apppndix V

lesign. fnr manufac turer ' s addre sses.

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:. 30MM ARTRIDGE CASE

Work

as initiated

ith the

stablishment

f the

asic

ize

nd configura-

tion f the 30mm ase ased pon the tated arameters. It as esired that the internal ropellant olume e at least 0 cubic nches nd the artridge case rim e 50mm n iameter. Using these uidelines nd including light external taper o id xtraction, an internal taper for asy mold core removal, and izing the eck for 0 m m , the asic hape as volved. Stress calculations redicted that uttresses in the oint ould e required to withstand the earing tress t he tooth urfaces imposed y the xpected case tensile loads.

A mold as designed nd constructed, drawings f hich (53395-1001)6 sheets 1 and ) are included in ppendix II f this report. The core hich forms the internal cavity as upported front nd ack to revent eflection

of the

ore during the injection cycle. The

molding

as accomplished

n a

New Britain 3-ounce capacity molding machine hich as a lamping force of 75 tons.

A an n arrel as fabricated n a ubcontract to Mathewson Tool Company, Orange, Connecticut. The 0m m arrel is 0 ''nches long ith ain twist rifling ranging from - 10 ' at the breech to 8 - 3' at the muzzle. The chamber is a eparate component threaded onto the arrel and the reech contained ercussion iring mechanism.

Based n the uccess at his time ith 12 ylon/43 ercent lass on the 20mm ase rogram, this material as chosen initially for fabrication f 30mm ase odies. The material erformed ell from he initial Mann arrel tests nd s herefore pecified or he inal esign.

A otal f 5 cartridges was test fired nder arious conditions during this evelopmental test rogram. In all tests, the ropellant was C1L 1379C, and the rojectile as the 30mm target ractice rojectile. T328, weighted to 5000 rains. The se f CIL 1379C ropellant as suggested y the government program manager ased pon atisfactory results ith this ropellant for other 0m m pplications.

The first hree tests ere charge development tests conducted to ascertain the correct amount f the ropellant. The charge weight was stablished at 160 grams. The results of the ext seven tests, conducted with the established eight of the ropellant, 160 rams, ere xcellent. There as no vidence of amage to the lastic/aluminum ase, or eparation etween the aluminum ase and lastic ody. The measured elocity averaged 3508 fps with xtremes f 3491 and 515 fps. All f these aiues are close to the muzzle velocity f 3500 fps hich as stipulated in the contract. The eak pressures in the un hamber ranged from 8,000 psi to 3,300 si ith an average of 0,000 si. These re ell within the eak ressures experienced

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in gu n chambers which normal ly experience peck pressures on th e order of 50,000 to 60,000 psi At this point, basic feasibi l i ty withia th e scope of th e contract had been established.

Although temperature extremes test ing was no t 1 requirement, i t was felt that addit ional test ing conducted at - 65 F would be useful in isolat ing possible problem areas f or future development . or these tests, th e Mann barrel chamber an d round were condit ioned at -65F as explained i n Section TIT.

It was immediate ly discovered that th e XM 11 5 primer would no t ignite th e propellant charge at -65F. A booster charge of on e gram of black powder

was added adjacent to th e XM 11 5 primer. The round was f ired at -65F an d th e pro jec t i le velocity was considerab ly higher at 4 2 7 4 fps. The case, however, showed no evidence of damage.

During these f irst tests, th e major concern was to ascer ta in th e integrity of th e plas t ic /a luminum case whi le a t ta in ing a prescribed velocity at ambient an d then l o ; ; temperature condit ions. However, on e recorded chamber pressure curve appeared to indicate an excess ive t ime delay between primer hi t an d peak pressure. hi le this observa t ion was incidental to th e purpose of th e test, the apparent time delay, neverthe less , was detrimental to autorat ic gun operation. The tine delay coupled with th e failure of th e X M 115 imer to ignite th e CTL 1 3 7 9 C propellant under low temperature condit ions prompted th e design of a booster which, therefore , could serve a dual purpose of

reducing th e time de lay an d e l iminat ing misfi re at low temperatures.

A booster tube assembly u t i i n g a No . Il l percussion primer an d a booster charge of 0. 3 gram of boron potass ium nitrate was designed and approved by A FAT L in June 1970. The booster assembly consisted of parts No. 5 3 5 9 3 - 4 0 0 0 4 , 5 3 5 9 3 - 4 0 0 0 8 , and 5 3 5 9 3 - 4 0 0 0 9 , th e detai ls of which are submit ted in Appendix IT of this report.

Seven ambient temperature and six lo w temperature tests were conducted using th e booster tube assembly. The ambient temperature tests showed an average project i le veloc i ty of 3 5 5 4 fps, slightly higher than th e average with th e X M 11 5 primer. Projec t i le veloc i t ies f or the various low temperatures were sl ightly below the 3508 fps average.

In al l th e above tests, no evidence of plast ic case cracking or separat ion f rom

AFATL-TR-72-401__Developement of 20MM and 30MM Plastic-Aluminium Cartridge Cases [1972]

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Transcript of AFATL-TR-72-401__Developement of 20MM and 30MM Plastic-Aluminium Cartridge Cases [1972]