I have chronographed 9mm loads with 3 and 4" pistols as well as the 16" Marlin Camp Carbine, and I was not pleased with the results.
The S&B 115 grain FMJ load was in FPS;
3"--1068
4"--1175
16"-1283
I attained the same results for the most part with my reloads that I worked up with AA#2 Imp. powder and 115 grain hornady fmj bullets loaded in S&B cases with WSP primers. I worked this load up to duplicate the 115 grain S&B load that shot& functioned well in all my "nines"
I think one could handload to a better velocity in the Carbine length barrel as the ammo available today is probably made to be efficient in pistol length barrels. The question would be, are the receivers in the Carbines built to withstand higher pressures or built just strong enough to handle the commericial ammo available?