Caliber
In small arms, caliber (American English) or calibre (British English) is the diameter of the inside of the barrel. This distance is measured from between the 'lands' in a rifled barrel, and is measured in inches. The caliber is expressed in either hundreths or thousanths of an inch, so .22 caliber is 0.22 inches. Other quite popular calibers are .357, .38, .44 caliber.Outside England and the U.S., the caliber of a weapon is more frequently expressed in millimeters (mm). Though it is technically incorrect to say 9 mm caliber, if the term caliber is used to mean 'barrel inside diameter', one could say 'the caliber is 9 mm'.
The caliber of a weapon is often informally used as a description of the type of arm it is. That is, a .45 caliber pistol is called a '45', a 9mm semiautomatic revolver is called just a '9 mm', a .380 caliber is called a '380', etc.
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.
See also: Table of calibers