calibers

Caliber is a term used in firearms and artillery to describe the size of the bore and the bullets they fire. In practical terms, it is a measure of the internal diameter of a gun barrel, expressed in inches or millimeters, and it closely corresponds to the diameter of the projectile designed for that bore.

In rifles and handguns, caliber is usually stated as a numeric value such as 9 mm, .45 ACP, .308 (7.62 mm), or .22 LR. The bore diameter is defined by standards, and bullet diameters are chosen to fit the rifling and ensure reliable engagement with the bore. The caliber name commonly appears in the cartridge designation and serves as a shorthand for discussion, but it does not by itself specify case length, powder charge, or overall cartridge size.

In artillery, caliber has a related but broader meaning: it can denote the bore diameter, and it can also describe the barrel length expressed in multiples of the bore diameter (for example, a 52-caliber gun). The usage varies by region and tradition.

Caliber is also spelled calibre in British English and other contexts, but the underlying concept is the same. Common calibers range from small to large, such as .17 HMR, .22 LR, 9 mm, .45 ACP, .308/7.62 mm, 7.62×39 mm, 5.56×45 mm, up to .50 BMG, with metric equivalents used around the world. Understanding caliber involves distinguishing bore diameter, bullet diameter, and the broader cartridge family to avoid confusion with overall cartridge size or case capacity.