How would the presence of an integral suppressor affect barrel length calculations? Would the suppressor count as part of the barrel, even though it reduces muzzle velocity?
To be honest, with an integrally-suppressed firearm, I’d look for or try to derive actual data on the weapon’s muzzle velocity/energy, rather than try to base it off barrel length. The reason is that the integral suppressor, particularly with supersonic rounds, will override the effects of barrel length on the weapon’s muzzle energy.
For example, the MP5-SD is noted to drop the normally supersonic 9mm’s muzzle velocity 16%-26%, to below the speed of sound. Let’s call it 20%. Assuming we’re using the 116gr 9mm JHP, we’re talking a reduction of muzzle velocity to ~992 feet per second; from there, we can derive a muzzle energy of 344 joules, or 19 damage, a reduction of 4 points of damage from the 116gr 9mm JHP’s base of 23 damage*. Thus, you could pretty safely say the MP5-SD would take a -4 penalty to damage.
*before arbitrary cartridge adjustments.
Which weapon(s) did you have in mind?
I am designing a fictional rifle based partly on the De Lisle carbine. It has a bolt-action, but I may change it to semiauto.
Ah; a .45 ACP weapon could probably get away with either a -1 or a neutral modifier, since the M2 Army Ball already has a subsonic velocity. A glance at Wikipedia suggests neutral; the 8-inch De Lisle Carbine has the same muzzle velocity as a 5-inch barreled handgun, and of course since you can’t actually increase the round’s velocity much further without rendering it supersonic, even a full rifle would probably only net, at best, a +1.
Of course, since it’s fictional, you’ve got a bit more leeway in how it functions.