Calibres

Calibre, or caliber in American English, is a unit of measure used to describe the size of a firearm’s bore or its ammunition. The most common meaning is the interior diameter of the gun barrel, i.e., the width through which a bullet travels. Caliber is typically expressed in inches in some countries or millimetres in others. For example, a .22 caliber rifle has a bore near 0.22 inches, while a 9 mm pistol uses a bore close to 9.0 millimetres. In cartridge designations, the caliber is embedded in the name, as in 9x19mm Parabellum or .308 Winchester, indicating the approximate bullet diameter.

In rifled firearms, the groove diameter describes the bore’s actual width, including the grooves, and bullets are sized to engage the lands. The nominal caliber is a practical label rather than a precise fixed dimension; tolerances exist, and actual bullet diameters may vary slightly to ensure proper chambering and rifling engagement.

Caliber distinctions matter for safety and compatibility. Ammunition must fit the firearm’s chamber and match the rifling to ensure reliable function and accuracy. Reloading, chamber cleaning, and maintenance practices also rely on correct caliber information.

In artillery, the term caliber can also denote the length of a barrel expressed as a multiple of its bore diameter, such as a 155 mm L/39 gun, where the barrel length is 39 times the bore diameter.

Regional spelling differences persist: calibres is the British English form used in many parts of the world, while calibers is common in American English. The concept is central to firearm design, ballistics, and firearm safety.