The word caliber (American English) or calibre (British English) comes from the Italian calibro, itself from the Arabic quâlib, meaning mould. It designates the interior diameter of a tube or the exterior diameter of a wire or rod.
In architecture, the caliber of a column is its diameter. In electricity, the caliber of an instrument of measure is the maximum value it can measure. In nautical parlance, the caliber of a chain is the diametre of the metal rod used to make each chain link. Agricultural produce is also often ranked by caliber (diameter), for instance olives, peas or eggs. In typography, the calibre of a font designates the size of the eye of a character, neglecting any risers or descenders.
In firearms, the caliber is the diameter of the inside of the barrel. In a rifled barrel the distance is measured between the 'lands'. The measurement is in inches and the caliber (abbreviated to cal) is quoted as a fraction (hundredths or thousandths) of an inch, so a 0.22 inch smallbore rifle is .22 cal.
Other common small bore calibers are .357, .38, .44 cal.
Outside Great Britain and the U.S., the caliber of a weapon is commonly expressed in millimeters (mm). It is technically incorrect to say 9 mm cal. If referring to the inside diameter of a barrel, you should say 'the caliber is 9 mm'.
The caliber of a weapon is often informally used to describe certain common weapons. A .45 cal pistol is called a '45', a 9 mm semi_automatic pistol is called a '9 mil', a .380 caliber is called a '380', etc. This gives rise to the impression that "cal" is a unit of measurement ,equal to 1/100 or 1/1000 of an inch, whereas it is just an unfortunate figure of speech.
For historical reasons, the name of a cartridge is not always the actual caliber. As one example, the common 38 Special revolver actually fires a bullet 0.357 inches in diameter .
Note that caliber is not a good indicator of the power of a cartridge. Bullet weight and shape, powder capacity of the cartridge, length of the barrel are some of the many variables.
Small arms range in bore size from approximately .177 cal up to .50 cal. Arms used to hunt big game may be as large as .700 caliber. In the middle of the 19th century, muskets and muzzle-loading rifles were .58 cal or larger.
The length of the barrel (especially for larger guns) is often quoted in calibers. The efffective length of the barrel from breech to muzzle is divided by the barrel diameter to give a value. As an example, the main guns of the Iowa class battleships can be referred to as 16"/50 caliber. They are 16 inches in diameter and the barrel is 800 inches (over 66 feet or nearly 20 m)long. Eight hundred divided by sixteen is fifty.