20 mm caliber is a specific size of popular autocannon ammunition. The dividing line between smaller-caliber weapons, commonly called "guns", from larger-caliber "cannons" (e.g. machine gun vs. autocannon), is conventionally taken to be the 20 mm round, the smallest caliber of autocannon. All 20 mm cartridges have an outside projectile (bullet) diameter and barrel bore diameter of approximately . These projectiles are typically long, cartridge cases are typically long, and most are shells, with an explosive payload and detonating fuze.
Weapons using this caliber range from anti-materiel rifles and anti-tank rifles to aircraft autocannons and anti-aircraft guns.
Twenty millimeter caliber weapons are generally not used to target individual soldiers, but rather objects such as vehicles, buildings, or aircraft.
Each weapon is listed with its cartridge type appended.
(incomplete list)
The usual nomenclature of ammunition indicates the diameter of projectile and the length of the cartridge that holds it; for example, 20ÃÂ102mm is a 20 mm projectile in a 102 mm long case.
Though this designation is often assumed to be unique, this is not always the case, e.g. there are three different 20ÃÂ110mm types which are not compatible. These may be distinguished in that some cartridge designations may include additional letters or names as a suffix, e.g. the various different types of 20ÃÂ110mm might be distinguished as 20ÃÂ110mm Hispano, 20ÃÂ110mmRB and 20ÃÂ110mm USN.