Pooch is partly correct. The term "gauge" was used to define a standard but the standard was not "bore" or barrel diameter but related to the chamber. Thus, with the introduction of breech loaders and cartridges there was a need for standardization between cartridge size and chamber size. Thus the term "12 gauge" was used to make sure that the cartridge was matched to the size of the chamber otherwise there could be catastrophic consequences if the two did not match and a cartridge slipped beyond the chamber. "Bore" only determines the diameter of the barrels since a "12 gauge" gun can have barrels which are 13 or 14 bore.

Regards - Ian Forrester