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When referring to projectile weapons, the term caliber, somewhat confusingly, refers to projectile weapons in two ways:

  • The inside diameter of the barrel (e.g., 5 inches/127 mm)
  • The length of the barrel in calibers (e.g., a 5"/54 caliber gun has a barrel that is 270 inches long).

The inside diameter measurement is used for both small arms (e.g., rifles, pistols, etc.) and for larger artillery. While all militaries have gone to metric measurements for the caliber of new weapons, in English-speaking countries, it is still common to find the calibers of small arms expressed in decimal fractions of an inch:

  • .223 inch, a standard NATO assault rifle round: 5.56 mm
  • .308 inch or 7.62 mm with various case lengths:
    • 7.62 by 51 mm, 30-30 Winchester
    • 7.62 by 61 mm, 30-06 used in the American M1903 Springfield rifle
    • 7.62 by 51 mm NATO weapon, .308 Winchester in civilian use
    • 7.62 by 38 mm in the Kalashnikov AK-47 and its descendants
  • .50 caliber, a common heavy machine gun, is 12.7mm

Weapons first developed under a metric system may acquire a decimal designation as well, such as 9mm pistol ammunition being called .380.

Alternative shotgun notation

To complicate matters, shotguns are most often referred to by "gauge", or the number of lead ball projectiles, in a single round, that would weigh one pound. 10 gauge shotguns have the largest calibers of shotguns in common use, at a very wide .775 inch, and 12 gauge = .729 inch. Shotguns chambered for .410 ammunition, are being described with respect to caliber alone.

Alternative artillery notation

Barrel length in calibers is used only with respect to artillery and naval guns. For example, the American Iowa-class battleships had 16" 50 caliber guns; barrel length was 16x50 = 800 inches.

While the diameter measurement is now the most common measurement, until the Second World War, and especially during the days of sail, cannon were described in terms of the weight of their projectile, a 32 pounder being a heavy gun on a ship of the line. Those were spherical projectiles, and both the change in shape to the streamlined bullet, as well as high explosive filler, made "pound" notation hard to compare.

In WWII, the 2-pounder was a 40mm, the 6-Pounder was 57mm.The 17-Pounder was 76.2mm. The 77mm gun was actually 76.2mm. It fired the same projectile as the 17-Pounder but used a less powerful charge.