The 20 mm caliber is a specific size of cannon or auto-cannon ammunition.
There are few weapons (aside from shotguns, large game hunting rifles, and heavy caliber muzzleloading “rampart” or “wall guns” popular in the early mid 19th century European militaries) which have been built that fire projectiles between 12.7mm (such as the 12.7mm NATO) and 20 mm caliber, though there were several 13mm heavy machine guns used during WWII, such as the MG 131; the 14.5 mm caliber is still used by some Soviet machineguns such as the KPV and anti-tank rifles such as PTRS, PTRD, and NTW-20.
Traditionally, in most nations, 20mm is considered the cutoff point between a weapon classified as a machine gun and a weapon classified as an cannon; anything smaller than 20mm is considered a machine gun, although it may utilize explosive shells (this can vary, however: for example, in WWII, Japan classified any weapon over 11mm as a cannon; thus, in Japanese records, their 12.7mm Ho-103 aircraft gun is classified as an autocannon, and it used explosive shells to overcome its lower muzzle velocity compared to American 12.7mm/.50BMG weapons).
A very small number of anti-tank rifles have been produced in 20 mm and larger calibers.
20 mm caliber cartridges have an outside bullet diameter and inside barrel diameter of 0.787 inches (20.0 mm). Projectiles or bullets are typically 75 to 127 mm (3–5 in) long. Cartridge cases are typically 75 to 152 mm (3–6 in) long. Many but not all 20 mm rounds have an explosive filling and detonating fuze.
As an example, the 20×102mm has a 100-gram bullet fired at a muzzle velocity of 1,035 m/s (3,396 ft/s). For a simple slug round this is a muzzle energy of 53,567 joules (39,509 ft·lbf).