The 12 Gauge is 0.729 in, 18.5 mm diameter and is by far the most common. The caliber of shotguns is measured in terms of gauge (U.S.) or bore (U.K.). The gauge number is determined by the weight, in fractions of a pound, of a solid sphere of lead with a diameter equal to the inside diameter of the barrel. So, a 10 gauge shotgun nominally should have an inside diameter equal to that of a sphere made from one-tenth of a pound of lead as an example. (Wikipedia)