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, 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. 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.

In the metric system usually 2 numbers are given. The first is the caliber, the second is the cartridge case length (case only, no bullet). This holds for pistol and rifle calibers, such as the 9x19mm (more commonly known as 9mm Luger) and the 5.56x45mm. The name of the creator may or may not be attached to the cartridge. The suffix "R" designates a cartridge with a protruding rim, such as 7.62x54R.

In the English system the caliber is given in hundreths (.xx) or thousandths (.xxx) of an inch and generally includes the cartridge creator's name. However the number given may be the barrel diameter (as described above), the bullet diameter (which is several thousandths larger than the barrel), or the cartridge diameter. Common examples: .270 Winchester (barrel diameter), .308 Winchester (bullet diameter), .38 Special (cartridge outside diameter).

In the 19th century the "dash" system was commonly used with cartridges. It consisted of two numbers defining the caliber and the powder charge. Popular examples include .25-20, .44-40, .45-70, and .30-30.

One very popular cartridge is the .30-'06 Springfield, commonly pronounced "thirty-aught-six". In this particular case ".30" is the barrel diameter and "-'06" is the year it was introduced, 1906. This is the only cartridge to be numbered in this fashion, but it is worth mentioning because of the huge popularity of the cartridge among sportsmen.

There are several common calibers that, while offered in a variety of cartridges, refer to a specific cartridge. A '45' typically refers to a pistol that uses .45 ACP (Automatic Colt Pistol). A '9mm' refers to a pistol chambered in 9mm Luger. A '.223' refers to a rifle chambered in .223 Remington (the commercial name of the 5.56x45mm NATO military cartridge).

Note that caliber alone is not a good indicator of the terminal ballistics (commonly refered to as "stopping power") of a cartridge towards an organic target. Bullet weight and shape, powder capacity of the cartridge, and length of the barrel are some of the many variables which also have an effect.

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 .800 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 (the rear portion of the barrel that opens to allow ammunition to be loaded) to muzzle (the muzzle of a firearm is the end of the barrel from which the projectile will exit) 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, et cetera. 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. Counter intuitively, a numerically larger gauge indicates a smaller barrel: a 20-gauge shotgun require more spheres to equal a pound, therefore its barrel is smaller.