uAl-cHAmber ingle-bArrel rifle - Berger Bullets€¦ · Winchester target rifle was chosen for the...
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48 www.riflemagazine.com Rifle 285 W ebster defines an experiment as an opera- tion carried out under controlled conditions in order to discover an unknown effect. The most unusual project covered here involves a rifle barrel with a functional chamber at each end, chambered and headspaced for two different cartridges. I had reasoned that the barrel grooves of precise bullet di- ameter, along with the tapered lands that form the leade, would impart full bullet stabilization upon its exit right through the unrestricted open chamber neck and larger chamber area. I remained quite con- vinced that the rifled leade, terminating at the open case neck mouth, would serve as a more than ade- quate crown, yielding complete bullet stabilization with good accuracy. This would be limited only to the rifling twist and load components used. Moderate throat/leade erosion would have minimal effect on accuracy performance, particularly where throat free-bore was minimal, and little change in per- formance was anticipated as the barrel was switched from one chamber to the next. This presented a real issue in the minds of some shooters, who believed that even directional change of the bullet would cause real accuracy issues. Anyway, as a long-time experi- menter, no further motivation was required in moving forward with this most unusual project. I would do all the gunsmithing myself. The initial plan was to thread and chamber for the 6mm Remington at the normally used breech end, crown the muzzle end and actually try the barrel in READER RESEARCH by Norman E. Johnson DUAL-CHAMBER, SINGLE-BARREL RIFLE This facsimile of a sectioned barrel with inverted bullet vividly depicts the principles of the dual-chamber, single-barrel rifle. With the barrel blank between lathe centers, it is contoured and polished. A separate chamber will be reamed at each end.
Transcript of uAl-cHAmber ingle-bArrel rifle - Berger Bullets€¦ · Winchester target rifle was chosen for the...
48 www.riflemagazine.com Rifle 285
Webster defines an experiment as an opera-tion carried out under controlled conditions
in order to discover an unknown effect. The most unusual project covered here involves a rifle barrel with a functional chamber at each end, chambered and headspaced for two different cartridges. I had reasoned that the barrel grooves of precise bullet di-ameter, along with the tapered lands that form the leade, would impart full bullet stabilization upon its exit right through the unrestricted open chamber neck and larger chamber area. I remained quite con-vinced that the rifled leade, terminating at the open case neck mouth, would serve as a more than ade-quate crown, yielding complete bullet stabilization with good accuracy. This would be limited only to the rifling twist and load components used.
Moderate throat/leade erosion would have minimal effect on accuracy performance, particularly where throat free-bore was minimal, and little change in per-formance was anticipated as the barrel was switched from one chamber to the next. This presented a real issue in the minds of some shooters, who believed
that even directional change of the bullet would cause real accuracy issues. Anyway, as a long-time experi-menter, no further motivation was required in moving forward with this most unusual project. I would do all the gunsmithing myself.
The initial plan was to thread and chamber for the 6mm Remington at the normally used breech end, crown the muzzle end and actually try the barrel in
READER RESEARCH by Norman E. Johnson
duAl-cHAmber,single-bArrel rifle
This facsimile of a sectioned barrel with inverted bullet vividly depicts the principles of the dual-chamber, single-barrel rifle.
With the barrel blank between lathe centers, it is contoured and polished. A separate chamber will be reamed at each end.
49March-April 2016 www.handloadermagazine.com
normal fashion before proceeding with the 6mm PPC chambering. This would allow for establishing accuracy at this early phase of the experiment. The barrel used was a six-groove, one-in-10-inch twist Green Mountain 416 stain-less full-size, 1.20-inch diameter blank 27 inches in length, the only barrel immediately available at the time. First the nearly 9-pound barrel was turned down to just un-der an inch in diameter between the 3-inch chamber sections that would be turned to 1.145 inches, thereby reducing the barrel weight to a very manageable 5 pounds, 10 ounces. With the breech ends re-duced in diameter to 1.145 inches from the original 1.20 inches, it would be the same diameter as post-1964 Model 70 target bar-rels. This left ample barrel shoul-der surface after the tenons were turned to one inch diameter and a length of 0.710 inch for the 16 tpi thread pitch.
Dave Manson reamers were used to ream both chambers. I reamed and headspaced the 6mm Remington chamber beginning with the recommended breech end of the barrel. At this stage of the experimental process, I planned to temporarily crown the barrel and try it before chambering the 6mm PPC end.
A proven post-1964 Model 70 Winchester target rifle was chosen for the project. I had earlier con-verted this rifle to a switch-barrel, with two other barrels, and had precision pillar-bedded the action. To make extraction of the 6mm PPC case’s .441-inch rim diame-ter compatible, making and ret- rofitting a totally new extractor was necessary. A brass protective thread cap was then made to cover and protect the barrel threads of the front chamber.
A Talley one-piece Picatinny base and Talley rings were in-stalled on the Model 70 receiver. Installation of a Picatinny mount-ing system adds measurable bridg-ing rigidity to a rifle receiver, such as the Model 70 Winchester. This was borne out in earlier controlled tests on weaker, rail-type actions.
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Inspection of the newly formed 6mm Remington chamber revealed a sym-metrical chamber wall neck and leade as is the norm with my Manson reamer. The throat (leade) section of the barrel was lapped prior to the normal break-in session. A num-ber of loads were tested with the 6mm Reming-ton chamber, along with the shoot-and-clean rou-tine for each shot. Ac-curacy was really quite good, with most groups at or near minute of angle. I learned, using a number of longer bullets in the 95- to 105-grain weight range, that this barrel, with its one-in-10-inch twist, would not sta-bilize them. A few other bullets up to and includ-ing 90 grains were tried before cutting the sec-ond chamber, and some surprisingly good groups began to appear. With a representative accuracy pattern established for the 6mm Remington cartridge with a single chamber, the 6mm PPC chamber was then reamed.
The 6mm PPC tenon prepping and chambering went well, and I was soon at the shoot-ing bench again. Handloaded 6mm PPC cases chambered with good feel. So there I sat in anticipation, shooting the very first rounds through the other end of the du-al-chambered barrel with at least
A threaded bronze collar was used to protect the threads on the muzzle end of the dual-chambered 6mm Remington/6mm PPC barrel.
some trepidation. I wondered just where, with respect to point of aim, the first group would arrive on target. Seeing it right in the midst of the previous 6mm Rem-ington group settled a lot of har-bored disquietude, along with the less than encouraging opinions of
A Manson reamer was used to form the chamber for a 6mm Remington cartridge. A special roller bearing center rest made threading and chamber reaming much easier.
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The Model 70 Winchester push-feed bolt face with a special extractor was made and fitted to extract case heads of different sizes.
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Above, these two groups were shot following the switch from 6mm PPC to 6mm Remington using two different bullets – Berger 90 grain and Hornady 75-grain V-MAX. Not only was accuracy amazing, but also point-of-impact for the two cartridges was precisely the same. Below, this series of groups was made with the 6mm PPC cartridge and Berger 90-grain bullets while testing the forend bedding screw system. The four lower groups were all shot without forend screw pressure. The four upper groups were shot with varying bedding pressure, causing each group to rise.
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The Winchester Model 70 target/varmint rifle with dual-chambered barrel (6mm Remington and 6mm PPC) is fitted with a Sightron 6-24x scope mounted with Talley rings and a Talley Picatinny scope base.
ited. More than one pundit thought accuracy would be dismal as the barrel was reversed.
I had admittedly anticipated the unknown quality of the barrel, along with the effects bore fouling stria and friction coefficient rever-sal may have on grouping perfor-mance, and the reverse chamber serving as a crown might also en-ter the equation, but such was not the case. Many very encouraging groups resulted, depending on the bullets and load used. I continued to clean the bore regularly, but found it shooting even better af-ter about 300 shots. The bore par-ticularly liked some bullets and powders over others and did very well with the Berger 80- and 90- grain bullets, the 90-grain Lapua, the Hornady 75-grain V-MAX and
some critics. Looking upon the de-mands for precision in turning the entire barrel down, forming the tenon and threads, barrel joint/shoulder, chambers at axially op-posing ends, it seems little short of a minor miracle the bullets would strike this closely to point of aim from a different chamber at the re-verse end of the barrel. Odds are overwhelmingly against an occur-rence of this nature. Shooting the remaining bullets into a very re-spectable group was a satisfying experience. Proceeding with the second stage of testing, it would have been unmindful not to con-sider some of the comments solic-
Sierra 80-grain Blitz. A number of other bullets will be tried later, but at this stage the greater con-cern was to see just what the dual-chambered rifle would do. Excellent accuracy was really happening.
The one-in-10-inch twist bar-rel, which turned out to be one in 10.5 inches, would not regularly stabilize bullets over 90 grains, not even 95-grain Bergers or match grade 105- or 107-grain weights. I quite suspected this from the outset, however, and would have preferred a one-in-8-inch twist. I have sent for some of the Berger 88-grain FB varmint bullets, which should fare very well in these chambers, but they have not arrived in time for inclusion in this work.
As the dual-chamber experi-mentation was furthering in its evaluative process, I switched between the two chambers, com-piling all the information. I didn’t
53March-April 2016 www.handloadermagazine.com
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make a big issue of monitoring velocity with a close correlation between those loads chrono-graphed, but while vacillating be-tween chamberings, performance actually showed improvement with both cartridges, and the bore no longer showed copper foul-ing. The leades of both chambers demonstrated no significant ero-sion, nor did the proximal periph-eral margins of each respective chamber neck, which, of course, serve as barrel crowns.
While drawing closer to the early stages of development on this project, both cartridges were
delivering a level of performance quite beyond that originally antici-pated. It was a bleak February day with a temperature of 10 degrees as I decided to try a couple more loads through the 6mm Reming-ton, chambered with two Berger bullets. The first two groups at 100 yards, with 90-grain bullets ahead of 42 grains of H-4831, each dis-played groups of .535 inch. All 10 shots would have stayed within the 10-shot group. Then two loads with Berger’s 80-grain flatbase bullets ahead of 42 grains of Re-loder 17 and 43 grains of H-4831 were tried. These loads were just
These are the major components used in building the dual-chambered rifle.
over .5 inch as well. Then, just in time for inclusion here, some Lapua 90-grain Senar bullets ar-rived; four, five-shot groups aver-aged .301 inch with these.
Now I see the dual-chambered barrel from a broader perspec-tive. Converting such an unusual plan to successful reality clearly speaks for itself. A bullet of proper stability would not be disturbed in its final exit through the ter-minal chamber neck serving as a crown, nor its exit through the open chamber. The major outcome of this experiment hinged on these very premises. R
