They chamber those barrels with a long, tapering throat that allows you to use either size. I shoot mine with 115gr. .312" bullets for field pistol matches and 165gr. .308" bullets for big bore standing. It does fine with either. The twist rate is 1:12" and will stabilize a 175gr. boat tail or 200gr. plain base bullet with no problem. I wouldn't shoot .311 jacketed bullets out of it though, there's no need and probably would pressure up quickly.