Caliber

In small arms, caliber (in American English, calibre in British English and other versions of English) is the diameter of the bullet (projectile), originally in inches. It is often informally used as a description of the type of arm (e.g., .45 caliber pistol) or of a specific cartridge.

Note that caliber is only a very rough approximation of the power of a cartridge. Other variables that come into play include bullet weight and shape, powder capacity of the cartridge casing and peak operating pressures.

In large naval guns (cannons), caliber refers to the length of the barrel, expressed as a multiple of the bore diameter. So the great guns of the Iowa class battleships are properly referred to as 16inch/50 caliber as they have a 16 inch diameter bore and are 800 inches (approx 20 meters) long.