In guns, particularly firearms, but not artillery, where a different definition may apply, caliber is the specified nominal internal diameter of the gun barrel bore – regardless of how or where the bore is measured and whether the finished bore matches that specification. It is measured in inches or in millimeters. In the United States it is expressed in hundredths of an inch; in the United Kingdom in thousandths; and elsewhere in millimeters. For example, a US "45 caliber" firearm has a barrel diameter of roughly 0.45 inches (11.43mm). Barrel diameters can also be expressed using metric dimensions. For example, a "9 mm pistol" has a barrel diameter of about 9 millimeters. Since metric and US customary units do not convert evenly at this scale, metric conversions of caliber measured in decimal inches are typically approximations of the precise specifications in non-metric units, and vice versa.
Rifle cartridges: from left: 50 BMG • 300 Win Mag • 308 Winchester, 7.62 × 39 mm • 5.56 × 45 mm NATO • 22 LR
A 45 ACP hollowpoint (Federal HST) with two 22 LR cartridges for comparison
Side view of a Sellier & Bellot 45-cal ACP cartridge with a metric ruler for scale
A gun is a device designed to propel a projectile using pressure or explosive force. The projectiles are typically solid, but can also be pressurized liquid, or gas. Solid projectiles may be free-flying or tethered. A large-caliber gun is also called a cannon.
SIG Pro semi-automatic pistol
Battleship USS Iowa fires a full broadside from its nine sixteen-inch naval guns
A 'flying-cloud thunderclap-eruptor,' a proto-gun firing thunderclap bombs, from the Huolongjing.
The first firearm (a "proto-gun"), the fire lance, from the Huolongjing.