I've wondered about this too, and I have seen better fps out of conventionally rifled aftermarket barrels than out of Glock barrels. I'm not sure whether it's the difference of rifling, the better case support or a mixture of both. Just some random thoughts here, but I think the conventional rifling bites into the bullet more which can raise pressures slightly causing higher velocity, or that could be completely wrong.
Lets not forget twist rate too, I think my KKM barrels have quite a bit different twist rates than what Glock uses.
Also, in the loose(r) Glock chamber it allows the brass to expand somewhat, and while this way of thinking may be totally wrong, it also increases internal case capacity slightly (due to case expansion) which lowers pressure causing slightly lower velocity. An aftermarket bbl with good support doesn't allow this case expansion (or maybe I should say as much) to occur which helps out velocity a little. Just a few theories to be tossed around.
I began to wonder about this when comparing velocities out of my 3.1" Kahr CM40 vs the 3.5" Glock 27. The Kahr has very good case support, conventional rifling with a different twist rate and without fail the shorter barrel Karh posted higher velocity than the longer barreled G27 with all loads, some in excess of 50 fps faster.
Last edited by SDGlock23; 08-16-2013 at 09:20..