Hi, Chief:
Sierra and Speer .303 bullets go .311" and Hornadys go .312". Just one thing, watch for tight necks. This isn't likely a problem in a military rifle, since chambers are usualy oversized, but it could happen. Be sure a .303 bullet is a loose fit in a case fired in your rifle before resizing.
Note to newbies: Chief's rifle should have a .308" bore, but it's oversized, so he wants to try a bullet that's .003 oversized, or .311". That's fine, IF his chamber neck is loose enough. BUT, if his rifle has only .003" clearance between the chamber and cartridge neck with a .308" loaded, and he loads a .311" bullet, clearance is zero. If this cartridge is fired, the case neck can't expand and release the bullet properly, and the pressure goes up, maybe way up. He might have to reduce the diameter of his case necks with a case neck trimmer.
Bye
Jack