AR-15 Jig Instructions Jig Instructions ver 1.1j.pdfFor the first time user, the Modulus Arms jig...

AR-308/AR-10 Jig Instructions

www.ModulusArms.com October 20, 2014

www.ModulusArms.com | AR-308 Jig Instructions 2

Introduction

If your lower can accommodate an AR-308 upper, it is compatible with the Modulus Arms 80% AR-308

Lower Receiver Jig. The Modulus Arms jig utilizes the same standardized lower features designed to

center an AR-308 upper to properly locate the features that need to be fabricated, resulting in a

universal fit. A billet lower completed with the Modulus Arms 80% jig using basic hand-held power tools

is shown below.

For the first time user, the Modulus Arms jig offers increased safety (no milling with a drill press), a

simple and easy to use process, compatibility with commonly available hand-held power tools, and

reduced risk of failure.

For the experienced builder, the Modulus Arms jig offers universal compatibility, increased precision,

and reduced fabrication time. Modulus Arms also offers individual replacement parts which are

available for purchase at www.modulusarms.com.

The lower is easily completed with four straightforward steps to fabricate the: 1) the trigger slot, 2) the

rear shelf pocket, 3) the fire control pocket, and 4) the fire control pin holes (trigger pin, hammer pin,

and safety selector). To reduce the fabrication time and to improve the finish, a router with an end mill

is used to finish the cavity that is first hollowed out with a drill bit in the first three steps.

Once any of the required finishing features are started, the lower is considered a firearm, and all local,

state, and federal firearms laws will apply. It is your responsibility to check the legality of finishing and

owning an 80% lower in your area. Please make sure you are familiar with the safe operation of power

tools before using the Modulus Arms jig to complete your lower.

Modulus Arms Jig Part Description & Key Features

The Modulus Arms 80% Jig kit contains the following parts as shown in figure 1 and listed below:



1. Template

2. Side Plate (2x)

3. Drill Guide

4. Front Support

5. Rear Support

6. Buffer Support

7. Depth Gauge

8. Cap Screws [8-32 x ½”] (14x)

9. Side Plate Support Cap Screw [8-32 x 1.5”] (2x)

10. Rear Take Down Cap Screw/Nut/Washer [8-32 x

2.25”]

Figure 1: Modulus Arms Jig Kit Contents (in production kits, short and long screws are the same size)

Unlike conventional 80% jigs, all of the features that require milling with the ¼” end mill are contained

on a single template that is designed to be used in two orientations (figure 2). The fire control pocket is

fabricated in one orientation while the rear shelf pocket and trigger slot are fabricated in the other

orientation. The template is attached to the front and rear supports allowing for proper centering of the

features. The side plates are attached with the oval holes allowing for a universal fit regardless of

variations in the lower receiver widths.



Figure 2: Template Features

To speed up the milling process, excess material is removed with a drill. The drill holes are located with a

drill guide installed under the template as shown in figure 3. Since the template only exposes a limited

number of holes in the drill template, there is no chance to drill a hole in the wrong location.

CAUTION: The drill guide is user friendly, one of the screw holes will not line up if the orientation is

incorrect. Furthermore, the drill guide cannot be installed upside down. For best results, start the drill

with the bit inside of the drill template to reduce the risk of accidental jig template damage.

Figure 3: Drill Guild installed under template in both orientations

The same side plate is used on the left and right side as shown in figure 4. The rear takedown hole is

provided to assure proper placement for the drilling of the safety selector and the trigger group pins.

The side plate support holes are designed to reduce flex and marring with anodized lowers when the

side plates are placed in a vise to secure the lower during the milling process.



Figure 4: Side Plate Features

The depth gauge is used for both drilling and milling as shown in figure 5. The gauge is used 1) to set the

drill depth using a drill stop on the 3/8” drill bit and 2) use to set the router depth for each pass and the

final pass. Depth gauge “A” is used to set the end mill so it exits the bottom of the lower for the trigger

hole. Depth gauge “B” is used for the rear shelf (depth = 0.630”). Depth gauge “C” is used for the fire-

control pocket (depth= 1.249”). The 1st hash mark is used to indicate the top surface of the lower. The

distance between the set of hash marks (approx. 1/8”) represents the maximum material that should be

removed for each milling pass. The bottom of the depth gauge is used to indicate the final pass. The user

is encouraged to start at 1/32” - 1/16” depth of cut and work up to a comfortable depth that should not

exceed more than ¼” depth of cut. Typically, a deeper depth of cut requires a router with variable speed

so the end mill speed can be slowed down for more control.



Figure 5: Depth gauge used to set the drill depth and end mill depth

The following parts are required to complete the jig and can be purchased as an optional kit: 1. ¼” x 4” 4-flute End mill with 1” Depth of Cut

2. 3/8” Drill Bit (fire-control, rear-shelf, and safety

selector)

3. 19/64” Drill Bit (trigger slot)

4. 5/32” Drill Bit (hammer/trigger pin)

5. 3/8” Drill Stop

Figure 6: Optional kit required to finish the lower

The following tools are required / recommended (examples shown in the figure below): 1. Router (full size or laminate)

2. Drill (hand drill or drill press)

3. Vise (drill press shown)

4. 3/64” Allen Wrench (for drill stop)

5. 3/32” Allen Wrench (8-32 cap screws)

6. Front Take Down (pin, detent, & spring)

7. C-clamp (optional if vise is not mounted)



Figure 7: Example of common tools that can be used to complete the lower (AR-15 jig shown)

Modulus Arms Jig: General Assembly Instructions

Front Support: The front takedown pin from your lower parts kit is used to secure front support as

shown in figure 8 in the same way the pin is used to secure an upper.

Figure 8: Install front take down pin and use it to secure the front support like an upper

Rear Support: Thread in the buffer support (clockwise) until the buffer support is below the rear surface

of the lower (figure 9), then further turn clockwise until the two screw holes are at 12 o’clock. Cap

screws can be partially threaded into the buffer support for additional leverage. Using the cap screws,

attach the rear support (do not tighten the cap screws). Make sure the buffer support is recessed so that

the rear support touches the back of the lower for proper alignment.



Figure 9: Install buffer support and rear support

Template and Drill Guide: The template attaches to the front and rear support using four cap screws.

For milling, the template is used alone (figure 10, left). For roughing, the drill guide is used (figure 10,

right). The first three of four steps involves installing the template with the drill guide to rough out the

pocket, then removing the drill guide to mill the pocket. When properly oriented, the head of all cap

screws should be recessed below the jig template top surface. After the 4 screws that are used to

attached the template to the front and rear support, tighten the two screws that attach the rear support

to the buffer support (figure 9).

Figure 10: Template only (left) or Template & Drill Guide (right) installed

Side Plates: The side plates attach to the jig through the template with four small cap screws (figure 11,

right). The jig is designed to accommodate different lower widths and should be pressed against the

lower before tightening the screws. For the first three steps, the main purpose of the two side plates is

to allow a vise to secure the jig, so the screw through the rear take down hole is not needed. For the

fourth step, the side plates serve as the template for the fire control group pins holes. For increased

accuracy, the rear take down screw/nut can be installed through the rear-take down hole. Unlike the



AR-15 design, the nubs on the side plate are designed to go into the rear take down holes in the lower.

The nubs are designed for a tight fit, the screw and nut can be used to snug the side plates against the

lower. The side plate support screws are designed to keep the side plates from flexing when clamped in

a vise. To accomplish this, the side plates are threaded on both sides, please use care when starting the

threads into the second plate.

Figure 11: Side Plate Installation

6061 vs 7075 Lower: Commonly, most forged lowers are made with 7075-T6, and billet can be offered in

either 6061-T6 or 7075-T6. In general, 7075-T6 is more difficult to work with requiring shallower depth

of cuts when using the router. Although a hand drill can be used with 7075-T6, a drill press can

significantly reduce the overall effort. If a hand-drill is used with 7075-T6, please be patient and go slow.

Tool setup: For best results, Modulus Arms recommends that the router and optional drill press is

properly setup to be square. Adjustments, if necessary, should be made before beginning the fabrication

process. If using a hand-drill, care must be taken to drill each hole in a square fashion.

Step 1: Fully Supported Trigger Slot

Configure jig for drilling: The template is installed with the rear-shelf pocket template closest to the

buffer tube mount as shown in figure 1.1 with the drill guide installed under the jig. The trigger slot

should have two drill holes and the rear shelf should have three drill holes as shown.



Figure 1.1: Jig configured to drill he rear pocket and trigger slot

Trigger Slot Drilling: With the two exposed holes in the trigger slot, use a 19/64” drill bit to drill all the

way through the lower (figure 1.2, left). Figure 1.2, right shows the lower after completion. For best

results, start the drill after the drill bit is partially or completely through the drill guide to prevent

accidental damage to the template. Make sure the hole is drilled normal (perpendicular in the x and y

direction) to the lower surface.

Figure 1.2: Drilling the trigger slot holes (left), lower after drilling (right)

Configure jig for milling: Remove the rear support and side support screw (6x) and pivot the jig template

up as shown in figure 1.3 (left). Remove the drill template (figure 1.3, middle). Reassemble using the two

rear support and four side plate screws. The properly configured jig is shown in figure 1.3, right.

Figure 1.3: Jig configured to mill the rear pocket and trigger slot



Trigger Slot Milling: A router and ¼” end mill is used to fabricate the trigger slot (figure 1.4, left) and the

completed trigger slot is shown in figure 1.5, right.

1. In figure 1.4 (middle) the end mill depth is adjust to the 1st template hash mark. This represents

the top of the lower. The maximum depth of cut is the distance between two hash marks.

Modulus Arms suggests that the user starts below this level and work up to a comfortable depth

of cut that never exceeds the maximum depth of cut.

2. Insert the end mill through the template and into either drill hole formed earlier. Make sure the

end mill does not touch the wall of the drill hole. Carefully turn on the router and allow the

router to come up to full speed.

3. With firm downward pressure applied to the base, mill using shallow clockwise passes with light

pressure. The 1st goal is the make shallow clockwise passes is to first adjoin the two holes. Next,

with light clockwise passes, widen until the end mill touches the side of the template. Turn off

the router, wait until the end mill stops spinning, and remove from the jig. Figure 1.5, left shows

the lower after the first pass.

4. Increase the cutting depth using depth gauge as a guide and repeat steps #2 and #3 until the

end mill breaks out of the bottom of the lower. The “A” depth gauge can be used to set the final

depth as shown in figure 1.4, right.

Figure 1.4: Router to mill out trigger-slot ¼” at a time (left) and final cut depth set with “A” depth gauge

Figure 1.5: Example of partially milled trigger slot (left) and complete trigger slot (right)



Step 2: Rear Shelf

Configure jig for drilling: Re-install the drill template using the reverse of process shown in figure 1.3 to

install the drill template. The jig should be configured as shown in figure 2.1. Remove (if present) the

rear takedown screw and nut.

Figure 2.1: Jig configured to drill out the rear pocket and trigger slot

Rear-Shelf Pocket Drilling: Set the drill depth on the 3/8” bit using the depth gauge as shown in fig 2.2

(right) then drill out the three holes exposed through the rear-pocket template to the indicated depth as

shown in figure 2.2. For best results, stop just before the drill stop touches the template surface. The

finished lower is shown in figure 2.3. For best results, insert the bit into the drill template holes before

starting the drill to prevent jig template damage

Figure 2.2: Drilling the trigger slot holes



Figure 2.3: Lower shown with 3 holes drilled for the rear shelf pocket

Configure jig for milling: The configured jig is shown in figure 2.4 after the drill guide is removed.

Figure 2.4: Jig configured to mill out the rear pocket and trigger slot

Rear-shelf Pocket Milling: A router and ¼” end mill is used to form the rear pocket (figure 2.5, right) and

the completed lower with the completed rear-shelf pocket is shown in figure 2.6.

1. Figure 2.5 (left) shows the end mill adjusted to touch the top of the lower as indicated by the 1st

hash mark. The distance to the next hash mark is the maximum depth of cut. It is suggest that

the starting depth of cut be made below the maximum depth and increasing the depth until one

reaches a safe and comfortable depth of cut that never exceeds the maximum. Figure 2.5

(middle) shows the end mill depth set for the final pass to achieve the targeted depth of 0.630”.

2. With the router set for the 1st pass depth, insert the router through the jig and into any of the

three previously drilled holes without the end mill touching the drill hole wall. Carefully turn on

the router allow the router to come up to full speed. (figure 2.5, right)


pressure. First adjoin the three holes by removing the rib between the holes, then widen with



shallow clockwise passes until the end mill touches the side of the template. Turn off the router,

wait until the end mill stops spinning, and remove from the jig.

4. Check the depth using the “B” depth gauge after each pass to make sure that the collet is tight

and the end mill is not pulling out. Repeat steps 2 and 3 with the end mill increment by the

desired depth of cut (not to exceed the maximum) until the final end mill depth shown in figure

2.5 (middle) is reached.

5. Figure 2.6 shows the lower before the 1st pass (top left), after the 1st pass (top right), after

several passes (bottom left), and after the final pass with the jig template removed (bottom

right)

Figure 2.5: Milling the rear-shelf pocket (left) “B” depth gauge at the maximum depth for a 0.630” rear pocket

Figure 2.6: Finished rear-shelf pocket



Step 3: Fire-Control Pocket

Configure jig for drilling: The template is installed with the fire-control pocket template closest to the

buffer as shown in figure 3.1 with the drill guide installed as shown. If the holes for the drill template

does not line up, it’s likely that the drill guide is orientated in the wrong direction.

Figure 3.1: Jig configured to drill the fire-control pocket

Fire-Control Pocket Drilling: The four holes exposed through the rear-pocket template are drilled out

with a 3/8” drill bit (figure 3.2, right) to a depth set with the “C” depth gauge (figure 2.2, right). Finished

lower is shown in figure 3.3.

Figure 3.2: Drilling the Fire-Control Pocket Holes



Figure 3.3: Lower with Fire-Control pocket holes drilled

Configure jig for milling: The configured jig is shown in figure 3.4 after the drill guide is removed.

Figure 3.4: Jig configured to mill the fire-control pocket

Fire-Control Pocket Milling: A router and end mill is used to form the cavity (figure 3.5) and the

completed lower with the fire-control pocket is shown in figure 3.7.

1. Figure 3.5 (middle) shows the end mill depth adjusted to touch to top of the lower as indicated

by the 1st hash mark. The distance to the next has mark is the maximum depth per pass. Figure

3.5, left shows the end mill adjusted for the final pass that will from the pocket to a depth of

1.249”

2. Insert router through the jig into a previously formed drill hole or trigger slot without the end

mill touching the side wall of the lower. Carefully turn on the router, and allow the router to

come up to full speed.


pressure. First adjoin the all holes and trigger slot by removing the rib between them, then

widen until the end mill touches the side of the template. Turn off the router, wait until the end

mill stops spinning, and remove from the jig. Figure 3.6 shows the lower after the first pass.



4. Check the depth using the “C” depth gauge after each pass to make sure the end mill is not

slipping. Increase the end mill depth by no more than the maximum depth and repeat until the

final depth is reached.

Figure 3.5: Milling the fire-control pocket (left) “C” depth gauge at the maximum depth for a 1.249” rear pocket

Figure 3.6: Fire-control Pocket after the first

pass



Figure 3.7: Completed fire-control pocket

Step 4: Fire-Control Pin Holes

The last step is to drill the holes which accommodate the fire-control trigger and hammer pin as well as

the safety selector as shown in figure 4.1. A finished lower is shown in figure 4.2.

1. Insert the rear take down screw through the side plates and tighten the nut.

2. Using a wooden block or metal parallels (in optional vise), support the side so the side plate is

parallel to the work surface.

3. Using a 3/8” drill bit, drill through only one side of the lower (figure 4.1, left) for the safety

selector (make sure the drill is perpendicular to the jig before drilling).

4. Using a 5/32 drill bit, drill through only one side of the lower to form the hammer (figure 4.1,

right) and trigger pin holes (figure 4.1, center). (Make sure the drill is perpendicular to the jig

before drilling)

5. Turn over the lower and repeat the process on the other side.

6. Run the 3/8” bit through both side pieces to make sure both safety bores are parallel to each

other



Figure 4.1: Drilling the fire-control pin holes using the side plate as templates to locate the holes

Figure 4.2: Trigger/hammer pin holes and safety selector holes

Process Overview

Figure 5.1 illustrates an 80% AR-308 lower as purchased. Figure 5.2 illustrates a completed lower using

the four steps outlined above. Figure 5.3 shows a complete lower after the lower parts kit and

buffer/stock assembly installed.



Figure 5.1: 80% AR-308 Lower before starting the process

Figure 5.2: AR-308 lower after complete all for fabrication steps



Figure 5.#: AR-308 completed lower after installation of the lower parts kit and buffer tube/stock assembly

Helpful Hints

This section will focus on additional hints and suggestions to improve the quality of the lower. The hints

and suggestions may be useful to both the first time novice as well as experienced 80% lower

fabricators.

Finishing Power Tools: The Modulus Arms jig is compatible with a number of different tools that can be

used to complete the drilling and milling process. Please make sure that you can safely operate the

power tools, and understand the manufacturer’s product manual.

A hand-drill or a drill press can be used to complete the 80% lower using the Modulus Arms jig. The

hand-drill is the lowest cost option, and its portability allows it to be easily borrowed from a friend or

rented. A drill press is a rarer and more expensive option, but results in reduced build time and

increased precision.

Drill Press

o Set the drill-press speed to approximately 2500 rpm.

o Make sure the drill bit is square to the table in both the x and y directions before using.

Hand-Drill



o Using the low (2-speed) or medium (3-speed) gear option if available.

o Best results are obtained with a more powerful, corded drill.

o Cordless drills will work; however, more than one battery and additional cool down times

may be required.

o For 7075 alloy lowers, the alloy is significantly more difficult to drill – a drill press is highly

recommended.

o Marking or center punching the drill spot on the lower can help increase accuracy. Using the

3/8” drill bit as a center punch, square the bit in the x and y direction with a right angle tool,

and lightly tap the end of the drill bit with a hammer to mark the center.

General Guidelines and Recommendations

o Make sure the drill bits are sharp – sharpen if necessary. Dull drill bits will result in

considerably higher effort and excessive temperatures.

o To prevent damage to the template edges, start the drill when the drill bit is through the

drill guide. After drilling, stop the drill bit before moving to the next hole.

o Frequently clear the chips by pulling the drill bit up but not out of the drill guide. The space

between the drill guide and lower is designed to accommodate the chips.

o Clear the chips between the drill guide and lower after each hole to prevent chips from

accumulating and compacting.

o Cutting oil or WD-40 applied with a toothbrush can reduce the drilling effort.

o After each hole, use the drill guide to check the depth to make sure the drill stop did not

slip.

For the milling process, the Modulus Arms jig is compatible with all common methods such as using a

drill press, router, or mini-mill. Using a drill press as a milling tool is NOT recommended and can be

dangerous. A router is common tool that is affordable and commonly used, making them easy to borrow

or rent. A mini-mill is a significantly more expensive option but results a significant reduction in build

time, and an increase in quality for the experienced user.

Drill-Press as a mill (NOT RECOMMENDED)

Router

o A router commonly used for woodworking with a standard ¼” collet.

o The end mill must be a 4” long, ¼” diameter end mill. Four flutes with less than a 1” cutting

depth is recommended for best results. Longer cutting surfaces can damage the jig. Always

check with the first cutting pass that the cutting flutes are below the jig template surface.

o Routers with variable speed give the bests results, although fixed speed routers can be used.

o Beware of inexpensive routers with a lot of flex – they can result in non-parallel walls and

damaged lowers.

o The results in these instructions were achieved with a Bosch PR20EVSK Colt Palm Grip

Router with the speed set at 4.

o End mill cut in a clockwise fashion. Best results are achieved with light clockwise semi-

circles, trying to keep only ¼ of the cutting surface on the metal to be removed.



o This jig has been tested with a number of different routers.

Mini-Mill

o A mini-mill with a Modulus Arms jig in a machine vise can be used. The x-y table offers

additional control, improved surface finish, increased speed, and reduce fatigue.

o Start with the end mill in a drill hole and start the mill. Slowly move table front and back

without touching the jig wall to cut a slot. This must be done at a slow feed speed since the

end mill is cutting on both sides.

o Cut either toward the front or back without touching the jig wall, then move front to back

removing more material (should be done in about two passes on each side of the starting

channel).

o Carefully control both the x and y to generate a light pressure against the template for the

final pass.

General Guidelines and Recommendations

o The user should start with a shallow depth of cut to gain experience before increasing the

depth of cut to a comfortable level that should never exceed ¼”.

Variable speed router is high recommended when approach the maximum depth.

The lower end mill speed allows for greater control

o If the end mill has larger than a 1” cutting surface, the first few passes need to be made

without the end mill touching the end of the jig. Only once the cutting surfaces have cleared

the jig, should the end mill touch the template edges to obtain the proper finished

dimensions.

o The end mill when working properly will feel like its cutting through butter. When the effort

increases, it is possible the flutes are clogged with metal. This can be removed with a small

pair of needle nose pliers.

o The clogging effect can be reduced by



Frequently coating the end mill with cutting oil or WD-40 applied with a toothbrush.

If possible, reduce the speed of the end mill.

Misc. Tools: The following are suggestions based on our experience.

Vise: The vise and work surface must not move or flex when routing. The vise needs to be either

clamped or bolted to the work surface. A drill vise shown in the figure was selected due to its low-

cost, common availability, and low profile that reduces flexing.

Shop-Vac: Great at removing chips and cleaning up the work area to prevent spreading of metal

chips

Toothbrush: Can be used to apply a small amount of oil on the end mill.

AR-15 Jig Instructions Jig Instructions ver 1.1j.pdfFor the first time user, the Modulus Arms jig...

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