Muzzle velocity is also affected by the amount of time and pressure by which the barrel is locked together with the breech face in the slide as the gases complete pressure.
As an added note, the rotating barrel system of the PX4 keeps the slide and barrel locked together for a longer time and movement than standard actions.
I don't recall chronographing 9mm Beretta velocities, but like it or not, Beretta barrels may be "slower". I've tested a bunch of factory and handloaded ammo in a variety of other 9mm pistols and revolvers. Yes, some barrels tend to be faster or slower. For instance, chronographing 9mm in SIG P226s and Glock 17s with similar length barrels. The Glocks are routinely faster, no matter the bullet weight, or standard pressure, +P, +P+, etc. The SIG P210 4.7" barrels tend to be just the opposite, i.e., faster. This, whether of Swiss, German or American manufacture, all tend to be faster than than "most" of my 5" 9mm 1911 barrels. I say most, because one 1911 Barsto 5" barrel was faster than the P210 SIG barrels. I have a S&W semi-auto with 3.5" barrel that almost always produces higher velocities than a similar S&W with 4" barrel. Thing is, everything matters, not just barrel length, twist rate, etc. No two manufactured items are exactly the same. Minute internal differences in barrels, like chamber dimensions, rifling leades, bore and groove diameters, internal finish, you name it, make a difference. This, even with same length barrels by the same manufacturer. Interesting stuff in any case, IMHO..
Please....
That's the theory I've heard. I've always been skeptical and have always thought it's just tighter lockup and better fitting of the properly fitted Apex barrel and slide locking points. I've got multiple M&Ps of both generations. They do seem to unlock early compared to almost anything else, but the barrel does remain locked to the slide for what would seem plenty long enough for the bullet to have left the barrel. I even have one of the very early M&P 1.0 Pros with it's 5" barrel which were reportedly one of the worst. Bullseye guns they are not, but I don't think it's unlock timing inconsistencies being the culprit.
My 2.0 Compact is pretty accurate. To the point I'm not doing an Apex barrel for the foreseeable future. This target is at 15-18'ish yards. Ignore the ones out of the 10 ring, I was dialing in my RDO.