410 or if memory severves 36 bore is a nominal size as is 12 bore. It may well be that two shotguns of the same caliber from the same manufactuer have differing bore sizes. In old English proof size there was only .005 between the sizes of a "410" and many were actualy proofed at anything btween .400" & .415".Makers try to standardise there prduction but if a barrel becomes "over size" ,why throw it away when it can easily be bored up to the next size?