I have a new .308 T/C Encore and it has yet to produse tight groups at 100 yds. I strictly followed a "break-in" proceedure of cleaning after every shot, then after every 5 shots, then 10 and so on.
I checked with a couple bench shooters I know who love the .308 and they said it could take 50 or more rounds in their experience to get tight groups.
So be patient, clean it a lot, and keep shooting. You can't shoot too much.
As far as wearing out a barrel . . . maybe in the magnum calibers . . . that's one of the reasons bench shooters don't use them. That and shoulder fatigue.