How do you calculate what gauge a shotgun is?

Answer 1

The gauge of a firearm is a unit of measurement used to express the diameter of the barrel. Gauge is determined from the weight of a solid sphere of lead that will fit the bore of the firearm, and is expressed as the fraction of a pound (e.g. a 1/12th pound ball fits a 12-gauge bore). The term is related to the measurement of cannon, which were also measured by the weight of their iron round shot; an 8 pounder would fire an 8 pound (3.6 kg) spherical cast iron ball and had a bore diameter of about 91 mm (3.6 inches). ( I do love metric measurements where the diameter in millimeters is the measure of a gun's "gauge")