Barrels can be shot out. Overbore cartridges like the 300 wthby are prime suspects. Barrels can go "south" in a hurry too. I wouldn't expect it to go after 300-400 rounds unless your friend shoots alot without letting the barrel cool. Hot burners like the wthby usually copper foul like crazy. Try really cleaning the bore with Sweets 7.62 copper solvent. check the barrel channel to make sure that its free floating.