Dritfrider explained this real well . . . . the copper jacketed bullets going down the steel barrel can eventually wear it out, but I'm guessing it would take 50,000 rounds. The throat and rifling immediately ahead of the throat is where the erosion takes place.

I built a local prairie dog shooter a .221 Fireball bench gun about 14 years ago. We used a full profile blank 27" long. About every two years, he comes back with a shot-out gun. Each time, I set the barrel back about 2.5"; re-thread it and re-chamber it. It seems to shoot as good or even a tad bit better each time we do this. Last time around, the length was just under 18" - so its about last dance for this Shilen 1:14 - that now has close to 25,000 rounds through it.