Gauge is a unit of measurement used to state the interior bore diameter of a shotgun barrel. ¹ The unit is determined by the weight, written as a fraction of one pound, of a lead sphere that will fit into the bore. For example, a sphere made of 1/12th pound of lead is .729″ in diameter. The interior bore diameter of a 12 ga. shotgun barrel is also .729″.

Gauge is commonly abbreviated as ga., ga, or G.

The size of the .410 Bore shotgun is the common exception to the use of gauge in this manner. If the .410 Bore shotgun was identified by gauge, it would be a 67.62 gauge.

1. “Glossary ⇒.” Sporting Arms and Ammunition Manufacturers’ Institute. Ed. SAAMI’s Technical Committee. Web. 15-18 Jan. 2011.