For historical reasons, the name of a cartridge often gives an imprecise indication of its caliber. As one example, the common 38 Special revolver actually fires a .357 inch diameter bullet.

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, length of the barrel, and peak operating pressures.

Small arms range in bore size approximately from .17 caliber to .50 caliber. In historical times, through the middle of the 19th century, muskets and muzzle-loading rifles of .58 caliber or larger were used, but modern weapons of larger than .50 caliber are usually designated as cannons rather than small arms.

In large naval guns (cannons), caliber also 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 16"/50 caliber as they have a 16 inch diameter bore and are 800 inches (approx 20 meters) long.