When I was still in high school, I decided not to buy a used Colt AR-15 because it rattled and looked awkward with its carry handle sights. Not my best day as a gun maven. Flash forward thirty years and the AR platform is everywhere, not just with soldiers throughout the world, but in sportsmen’s pickup trucks and the skilled hands of competition shooters. The modular system designed by Eugene Stoner has really grown up and the variety is dazzling
The AR pistols generate quite a few questions, specifically regarding velocity loss from the much shorter barrels. Their owner’s estimates vary wildly from a few hundred feet-a-second less than the 24-inch test barrels to as much as a fifty percent loss. Like most things in life, the answers lie somewhere in the middle.
We took one of Rock River Arms piston-driven pistols out for a test of three popular bullet weights to see how much velocity was lost to its 8-inch barrel. The LAR – PPS is very accurate and fun to shoot once you get past the muzzle blast and sharp report. Without Stoner’s gas system, no buffer tube is required so the overall length is a short 17.5 inches. It weighs in at five pounds.
All of the tested loads used Ramshot TAC for a propellant along with Winchester cases and primers. Each of the load choices were made from our 5.0 Guide maximum loads for the .223 Remington and operated at or below the SAAMI maximum of 55,000 psi.
The first tested load used a 40 grain Sierra Blitzking using 27.6 grains of TAC. In our 24-inch pressure barrel, this load produced 3715 fps recorded nine feet from the muzzle. It was the most diminished round tested compared to the full-length barrel.
Average velocity from the Blitzking averaged 2532 fps and 570ft/lbs. of muzzle energy. The velocity loss in this case was 74 fps per-inch of barrel lost to the tested 24-inch barrel. The percentages tell the story more plainly. The pistol velocity decreased 32% over the rifle with a 53% loss of energy.
The Sierra Spitzer Boattail performed better from the pistol. This 55 grain bullet pushed by 25.5 grains of TAC averaged 3281 fps from the 24-inch test barrel with 1315 ft/lbs of energy. The pistol averaged 2327 fps and 662 ft/lbs, a decrease of 29% in velocity and 50% in energy.
77 Grain Nosler Test comparison between 24-inch rifle and 8-inch pistol.
The heaviest bullet tested was Sierra’s 77 grain Matchking which produced 2759 fps from the test barrel. The Rock River pistol developed 2059 fps from its 8-inch barrel. This load’s 25% decrease in velocity was the lowest of the group, along with its 44% loss in energy.
What Does it Mean Technically?
TAC is on the slower side of the useful range for the .223 Rem/ 5.56X45mm and we suspected that this would explain both the diminished performance of the lighter bullets as well as the better-than-expected velocity of the heavy 77 grainers. To test this, we used the same test platforms but with a faster powder. As expected, performance with the heavier bullets dropped off to about 1850 fps, but the lighter bullets, with the faster powder still lost out to TAC in velocity.
These results instead tend to emphasize how much velocity is introduced to the bullet in the first several inches of a rifle’s barrel. After that point it is gas volume that predicates muzzle velocity and that volume is enhanced by larger charges of powder. That seems to hold true, given the same pressures, even in pistol barrels. The faster powder, shorter barrel idea that is so intuitively correct for handloaders and ballisticians, was not borne out in this test.
Loading for an SBR or AR-Pistol
In an AR platform, gas volume either drives a piston or impinges directly on the bolt carrier group. The little piston Rock River Arms didn’t like the light bullet/fast powder combination and showed signs of being under-gassed. It often refused to lock back on an empty magazine.
The same bullets using TAC locked back consistently. This isn’t because the loads were “hotter”. They all operated at the very similar pressures. It was because the higher charge of the slower burning powder produced more gas to drive the action. It is simple: Gas Volume Drives ARs.
It looks like a good load in your AR-15 rifle will be a functional load in your AR-pistol but don’t bet the farm until you test it for reliability. Not all firearms are created equal. If you have one that doesn’t want to cycle on your pet handload, the best bet is to move to a slower burning powder. If that fails, increase the bullet weight. After that point it may be necessary to play gunsmith and exchange the buffer for a lighter one, but do that last after switching powders to increase gas volume. Oh, and wear your ear muffs. These things bark.