Caliber

The word calibre (British English) or caliber (American English) designates the interior diameter of a tube or the exterior diameter of a wire or rod. It comes from the Italian calibro, itself from the Arabic quâlib (قالب), meaning mould.

The term most often appears with respect to firearms, as a measure of the size of the barrel; however, it also has use in other fields.

Firearms

In firearms, the caliber is the diameter of the inside of the barrel. In a rifled barrel the distance is measured between the lands.

If the measurement is in inches then the caliber (abbreviated to cal) is quoted as decimal of an inch, so (smallbore) rifle with a diameter of 0.22 inch is a .22 cal ("twenty-two caliber").

Calibers of weapons can be referred to in metric, as in a "caliber of eighty-eight millimetres" (88 mm) or "a hundred five-millimetre caliber gun" (sometimes abbreviated as '105mm gun').

Small arms range in bore size from approximately .17 cal. up to .50 cal. Arms used to hunt big game may be as large as .80 caliber. In the middle of the 19th century, muskets and muzzle-loading rifles were .58 cal or larger.

Caliber as measurement of length

The length of the barrel (especially for larger guns) is often quoted in calibers. The effective 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 long (16 * 50 = 800). This is also sometimes indicated using the prefix L/, so for example, the most common gun for the Panzer IV tank is described as a "75 mm L/48", meaning a barrel 75 mm in diameter, and 3600 mm long.

Alternative measurements of bore

Measurement of the bore of large weapons can be, and often was, expressed in pounds. The weapon would be named according to the weight of a sphere of lead of the same diameter as the bore. The density of lead was used because it is a traditional material for projectiles.

This leads to certain guns being referred to as 6-pounder, 25-pounder and so forth. However this relationship between calibre and projectile weight changed with the introduction of the cylindrical rifled shell. The gun continued to be named by the weight of projectile it threw although this no longer gave a direct indication of the barrel size.

Shotguns are named according to gauge, a related expression. The gauge of a shotgun refers to how many lead spheres the diameter of the bore would equal a pound. In the case of a 12-gauge shotgun, it would take twelve spheres the size of the shotgun's bore to equal a pound. Counterintuitively, a numerically larger gauge indicates a smaller barrel: a 20-gauge shotgun requires more spheres to equal a pound, therefore its barrel is smaller.

Rockets and Missiles

Dimensions of rockets and missiles, particularly the location of the centre of pressure, are usually expressed in calibres, where one calibre is equal to the body diameter.

Aviation bombs

Some countries (USSR and Russian Federation, for instance) use the "caliber" term to classify aviation bombs. The Russian/Soviet bomb caliber is expressed in mass/weight units, but may not be equal to the mass/weight of the munition.

Other uses

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 diameter 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 caliber of a font designates the size of the eye of a character, neglecting any risers or descenders.

In horology, the term is used to distinguish the size and type of movement used within a timepiece.