Years ago, we were told that velocities were estaqblished in 26" barrels, and the "rule of thumb" was 25 fps per inch. I graphed factory loads in my Rem 742 (22" barrel) and got 2600 fps which was bang-on the estimate. I later bought a 760 carbine (18 1/2" barrel), and found I would get identical velocities to what I'd got from my 742. The "rule of thumb" would have said those velocities should have been 85-90 fps less that what I actually got.
Based upon this, and other observations over the years, I've found myself adhering to the truism that every gun is a law unto itself, and that you have to shoot it to know what the effect of barrel length might be.
Logically, it makes sense that in an autoloader, since you bleed off some of your expanding gas to work the action, this should have an effect on velocity. I've just never been able to quantify (or notice) it myself.
