Gauge (firearms)

Gauge
From left to right; a .45 ACP cartridge, a .410 bore shotshell, a 20-gauge shotshell, and a 12-gauge shotshell
Type	Shotgun
Place of origin	Various

The gauge (in American English or more commonly referred to as bore in British English) of a firearm is a unit of measurement used to express the inner diameter (bore diameter) and other necessary parameters to define in general a smoothbore barrel (compare to caliber, which defines a barrel with rifling and its cartridge).

The gauge of a shotgun is a list that includes all necessary data to define a functional barrel. For example, the dimension of the chamber, the shotgun bore dimension and the valid proof load and commercial ammunition, as defined globally by the C.I.P.; defined in Great Britain by the Rules, regulations and scales applicable to the proof of small arms (2006)^[1] of The London Proof House and The Birmingham Proof House, as referred in the Gun Barrel Proof Act 1978, Paragraph 6;^[2] and defined in the United States by SAAMI Z299.2 – 2019.