Could be a combination of everything mentioned above..I'd go with some serious copper fouling first especially since you mentioned Barnes bullets.. second the physical condition of the rifle, screws and the barrel crown...check to see if a hunk didn't get blowed out, it wouldn't be the first time.. what about a bore scope of the throat area, the .257 is known to give her some heat in short order...the scope internally could be damaged, Zeiss or no Zeiss it's happened to all of them from time to time.. Get youself a good copper cleaner and some JB paste and clean the heck out of her first and I mean really clean it out.. the rest is a matter of trial and error..