caliber

Caliber is a measurement used primarily with firearms to describe the approximate internal diameter of a gun's bore and the diameter of the bullets it fires. It is usually stated in inches, such as .45 or .22, or in millimeters, such as 9 mm or 7.62 mm. The caliber of a round is related to, but not always identical with, the cartridge or case; in practice the bullet diameter is sized to fit the bore. In firearms nomenclature, the term is also used to describe the overall round size rather than the case alone. In artillery, caliber has an additional meaning: the length of a gun's barrel expressed as a multiple of the bore diameter (for example, a 40-caliber barrel is 40 times the bore diameter). The word originates from French calibre or Italian calibro, from Latin calibrare, meaning to measure or gauge.

Separately, caliber is used in everyday language to denote quality or capability; a person of high caliber has notable competence or achievement. Spelling varies by region (caliber in American English, calibre in British English). Caliber is not to be confused with gauge, which is a distinct system for shotgun ammunition sizing. Examples include .308 Winchester, 9 mm Parabellum, and .22 caliber.