Caliber

When referring to projectile weapons, the term caliber, somewhat confusingly, refers to projectile weapons in two ways:

• The inside diameter of the barrel (e.g., 5 inches/127 mm)
• The length of the barrel in calibers (e.g., a 5"/54 caliber gun has a barrel that is 270 inches long).

The inside diameter measurement is used for both small arms (e.g., rifles, pistols, etc.) and for larger artillery. While all militaries have gone to metric measurements for the caliber of new weapons, in English-speaking countries, it is still common to find the calibers of small arms expressed in decimal fractions of an inch:

• .223 inch, a standard NATO assault rifle round: 5.56 mm
• .50 caliber, a common heavy machine gun, is 12.7mm

Weapons first developed under a metric system may acquire a decimal designation as well, such as 9mm pistol ammunition being called .380.

Barrel length in calibers is used only with respect to artillery.

To complicate matters, shotguns are most often referred to by "gauge", or the number of lead bal projectiles, in a single round, that would weigh one pound. 10 gauge shotguns have the largest calibers of shotguns in common use, at a very wide .775 inch, and 12 gauge = .729 inch. Shotguns chambered for .410 ammunition, are being described with respect to caliber alone.