Caliber

Caliber is the measure of the internal diameter of a firearm's barrel, or the diameter of the projectile fired from it. It is measured in millimeters or inches. For example, a firearm described as 9 mm refers to a projectile diameter of approximately nine millimeters; a firearm described as .45 refers to forty-five hundredths of an inch.

Caliber is one of the most important characteristics used to distinguish types of firearms and ammunition. Ammunition and a firearm of matching caliber will function together safely. However, caliber alone is insufficient to determine compatibility: each round also has specific cartridge length, primer, and construction.

How caliber is measured

Most barrels contain internal spiral grooves (rifling), which means the diameter has two forms: one measured at the raised sections (lands) and one at the recessed sections (grooves). For this reason, different countries and manufacturers may assign different numbers to the same firearm. This is why caliber designations are sometimes commercial or historical names rather than precise measurements.

Gauge system for shotguns

Shotguns are usually not referred to by caliber but rather by what is called 'gauge' or 'bore'. In that system, the number indicates how many lead balls of diameter equal to the barrel can be cast from one pound of lead. Consequently, a smaller number (such as 12) means a wider barrel than a larger number (such as 20). This is a system different from the standard millimeter measurement.

Artillery and large-caliber weapons

The term caliber is also used for artillery and naval weapons. There, it sometimes refers to the length of the barrel measured by how many times the internal diameter fits into it. For example, a gun described as '50 caliber' in length means the barrel length is fifty times the internal diameter. The meaning of the term thus depends on the context in which it is used.