Originally Posted by 57K
… With faster burners than W231 the difference will be pretty insignificant, but if you're using something from the medium-slow group like Silhouette, HS-6 or 3N37, you'll see a significant increase.
The difference won't be nearly that big. QuickLOAD offered up 4½% velocity difference for a very fast powder and 6½% for a very slow powder in this instance. The reason is added barrel length won't stop increasing bullet velocity until the barrel is so long that the pressure drops to where it provides only enough force to overcome bore friction and no more. Even for the .22 Long Rifle, which uses very fast powder and has a huge expansion ratio, velocity won't stop increasing with barrel length until you get a barrel 16-20 inches long. Normal pistol barrels are nowhere near long enough for that to happen, even with fast powder loads.
Now if you were to tell me you saw no difference in velocity between a particular short barrel gun and some other, longer barrel gun in the same chambering, that's perfectly believable. As I mentioned before, chamber-to-chamber differences can easily account for bigger velocity differences than the percentages I gave. One time I was testing some match ammo in different club-ownded Garands, and had individual rounds in one rifle differ by 200 fps from individual rounds in another rifle. That was about 8% difference just due to headspace, throat, and bore conditions in the rifles. In handguns the percent difference can be even bigger because each thousandth of an inch of chamber dimension difference represent a bigger percent difference in expansion ratio in a short straight wall cases than it does in a bottleneck rifle case.
So, unfortunately, the only way you can say for sure that extra barrel length is not getting you anything in your particular gun is to start cutting the barrel down and measuring the velocity change that results. Comparison to another gun won't tell the tale.