The 20 mm caliber is a specific size of cannon or autocannon ammunition, commonly the smallest caliber which is unambiguously a cannon (or more commonly today, autocannon) and not a heavy machine gun.
There are few weapons (aside from shotguns and large game hunting rifles) which have been built that fire projectiles between .50 caliber (0.50 inch/12.7 mm, roughly 13 mm caliber) and 20 mm caliber, though the 14.5 mm caliber is used by some Soviet machineguns such as the KPV and antitank rifles such as PTRS, PTRD, and NTW-20.
A very small number of anti-tank rifles have been produced in 20 mm and larger calibers.
20 mm caliber cartridges have an outside shell diameter and inside barrel diameter of 0.79 inch (20 millimeters). Projectiles or shells are typically 75-127 mm (3-5 inches) long. Cartridges are typically 75-152 mm (3-6 inches) long. Many but not all 20 mm shells have an explosive filling and detonating fuze.
As an example, the 20x102 has a 100 gram bullet fired at a muzzle velocity of 1,035 m/s (3395 ft/s). For a simple slug round this is a muzzle energy of 53,567 joules (or approximately 39,507 ft-lbf).
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Like most cannon ammunition, 20 mm caliber weapons are typically used against large targets such as vehicles, buildings, or aircraft. Though effective against individual soldiers, 20 mm ammunition is so large and heavy that its effects are nearly wasted on relatively small targets.
Each weapon is listed with its cartridge type appended.
Cartridge type indicates the diameter of projectile and the length of the cartridge that holds it, for example 20x102 is a 20 mm projectile in a 102 mm long case. Only rarely do two designers use the same case length, so this designation is usually definitive. Some cartridge types have additional letters or information about them listed.
