The bore of a gun is the inside diameter of its barrel. As a result, "bore" is sometimes used as a unit, meaning the same as gauge [1] for shotguns and the same as caliber [1] for other guns
Russ Rowlett (Director), How Many? A Dictionary of Units of Measurement, Center for Mathematics and Science Education University of North Carolina, USA 2005. http://www.unc.edu/~rowlett/units/dictA.html, (Unit of Meas and Covr Factors - Amer) {608}