I don’t find it to be a consistently linear matter, despite what I have been lead to expect.
Factory test barrels are 24”, unless otherwise noted.
My CZ 527 with a 24” factory barrel and my CZ 527 with an 18” PAC-NOR barrel and a 6” suppressor get average velocities of 2530 and 2509 with Hornady 6.5mm Grendel 123-grain ELDMs. The box claims 2580 FPS.
My 16.1” Tikka T3 .223 with a 7” suppressor gets higher velocities than the box velocity for about half the rounds I tested and slightly lower than box velocity for the other half. In one case, I was getting 150-200 FPS more than the box velocity (using 5.56).
My 20” Tikka T3X 6.5 CM with a 6” suppressor gets an average of 2560 FPS as opposed to the box velocity of 2710 with Hornady 140-grain ELDMs.
My 21” Mauser .270 with a 4” suppressor gets an average of 2885 FPS as opposed to the box velocity of 3060.
My 24” FN .270 got an average of 2887 with the same lot of ammo.
My 24” FN .25-06 with a 4” suppressor gets 2908 using Federal Premium 120-grain as opposed to a box velocity of 2980.
All these samples are with at least 20 cartridges from the same lot over the same Garmin on the same range (not the same day).
In short, while I was taught to expect that cutting down your barrel would result in losing 25-50 FPS per inch, my experience has not shown that to be universally the case.
I’d just go shoot a few different loads and see what your rifle shoots best. Then use that.
