Could it be possible that the chamber was modified to the .303 case dimensions (reamed to advance the shoulder), but the bore is still 30 caliber... but perhaps oversize.  It is possible to throw a .312 bullet down a .311 or .310 sized bore with little problems.  My guess would be that someone reamed the chamber to .303 dimensions because of the scarcity of 30-40 ammo and brass.