I've know an internet guy who shoots P-dogs a LOT and he whacks them out to 500 yards (lasered) with a .223. Pretty regularly. Beyond that, he goes to 6mm.
He wears out barrels with the 6mm, and probably with the .223, but he shoots a lot of rounds. I forget how many rounds it takes him, I'm thinking 4000-6000 rounds. He uses muzzle brakes on his rifles so he can see the bullet strike and adjust in case of a miss. I don't know if that has any affect on the barrel wear, would suspect it doesn't.
Being way overbore channels the heat down to a narrow opening (the bore) and cuts it away much quicker.
