I've got a 14 inch barrel and it shoots as well as, if not better than, my carbine length T/C barrel. In general, I can shoot 1.5 to 2 inches at 100 yards with multi-round strings when shooting from a rest. I have shot (played around) shooting at large targets as large as prairie dog mounds out to distances at and over 400 yards with fairly repetitive results. (That is, scoring hits on mounds of roughly 4X4 ft with repeatability when shooting off my knee in a sitting position)
I have a 4X scope on mine and it's a lot of fun to play with. When shooting the long distances, I'm half way below center on my scope crosshairs in order to compensate the distance.
While you say the round is inexpensive, compare 110 grain FMJ or soft point bullets with those of .357 magnum 158 grain JHP's. The price is the same. With the exception of a few grains of powder, you can load the .357 for pretty much the exact same price. So, overall, you can probably get yourself a .357 mag barrel a lot faster than waiting around until a .30 carbine barrel shows up. My .357 pistol barrel is just as accurate as the .30 carbine pistol.