Caliber

There are two recognized definitions of caliber that differ slightly. First, there is the factual term caliber that is defined as the internal diameter of the barrel of a firearm. Second, the nominal caliber of a bullet refers to its nominal diameter and the characteristics of the cartridge. In many instances, the numerical value of the nominal caliber corresponds to the factual caliber, but this is not always the case, as some variations might appear.

The caliber of a cartridge can usually be determined from the size, weight, and shape of the bullet. Once the caliber of a cartridge is known, it is possible to obtain a list of firearms capable of firing such a caliber. In addition, by looking at other characteristics imprinted on the bullet, it is possible to further narrow the possibilities. This is extremely important evidence in a forensic investigation. With one bullet found at a crime scene it is possible to determine which firearms could have fired the bullet, and therefore, enhance the search for a suspect.

Calibers are usually expressed in hundredths of an inch or thousandths of an inch in the United States and in Great Britain. In many other countries, these are expressed in millimeters. For example, the .25 Auto is equivalent to the 6.35 Browning, the .32 Auto is equivalent to the 7.65 Browning, and the .30 Luger is equivalent to the 7.65 Para. This can be confusing if the conversion is unknown.

Calibers range from very small to very large. Among the commonly known calibers, one of the smallest is the .117 or 4.5 mm and one of the largest is the .700 Nitro Express. Some well-known calibers are the .22 LR (long-rifle), 9 mm Para, .38 Special, .357 Magnum, 10 mm Auto, and .45 ACP (automatic Colt pistol).

The term gauge instead of caliber is used with smooth-bore firearms such as shotguns. In this case, the gauge number corresponds to the number of identical spherical balls of pure lead fitting the gun's diameter that could be made from one pound of lead. For example, a 20-gauge shotgun has a diameter of 15.6 mm, meaning that one pound of pure lead can make up 20 identical balls of a diameter of 15.6 mm each. Gauge numbers range from 4 to 26, which corresponds to 23.4 mm and 10.2 mm, respectively.