I have two 20 inch A2 profile barrels I purchased from Bushmaster earlier this year. I just got around to installing them on two upper receivers last week, and guess what--they both short-stroke!
I have six AR's, a mixture of Bushmaster, Colt, and PWA lowers(all but one pre-ban), in a variety of configurations. I have replaced the gas tubes and tried different recoil springs, bolt carriers, and have mixed and matched the new Bushmaster A2 barrels with different lowers(except the post-ban lower--that would be illegal), and the conclusion is inescapable--they short-stroke consistently. The bolt usually cycles rearward just enough to eject the empty brass, but not far back enough to pick up a new round, or lock back on an empty magazine. Or sometimes the bolt doesn't even go far enough back to eject the brass, and the action locks up with the spent brass stuck at a cockeyed angle in the upper receiver.
As a test, I tried exchanging a short carbine buffer for the full length rifle buffer and I found that both rifles would then function normally. This of course is WRONG-and maybe not a smart thing to do. But clearly there is an inadequate gas impulse delivered to the bolt carrier to overcome its inertia.
I suspect that the gas ports in the barrels are too small.
Opinions, suggestions, recommendations, anyone?