The gauge of a firearm is a unit of measurement used to express the inner diameter of the barrel.
From left to right; a .45 ACP cartridge, a .410 bore shotshell, a 20-gauge shotshell, and a 12-gauge shotshell
left-to-right: .410 bore, 28 gauge, 20 gauge, and 12 gauge shotgun shells
Garden gun calibers: 9mm Flobert shot, 9mm Flobert shot, .22 Long Rifle shot, .22 Long Rifle, .22 Long Rifle shot, .22 CB Short, and 9mm Flobert BB cap
A 10-gauge (3+1⁄2-inch (89 mm)) shotgun shell shown next to a United States quarter
A firearm is any type of gun that uses an explosive charge and is designed to be readily carried and used by an individual. The term is legally defined further in different countries.
The Colt Single Action Army, a revolver chambered in .45 Colt
The Glock 19, a pistol chambered in 9×19mm Parabellum
The Mosin–Nagant, a rifle chambered in 7.62×54mmR
The M4 carbine, a carbine derivative of the M16 rifle, chambered in 5.56×45mm NATO