20 mm caliber

The 20 mm caliber is a common firearm bore diameter, typically used to distinguish smaller-caliber weapons, commonly called "guns", from larger-caliber "cannons" (e.g. machine gun vs. autocannon). All 20 mm cartridges have an outside projectile (bullet) diameter and barrel bore diameter of . 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.
Usage.
Twenty-millimeter-caliber weapons are generally not used to target individual soldiers, but against targets such as vehicles, buildings, or aircraft.
20 mm weapons.
Each weapon is listed with its cartridge type appended.
Historical weapons.
Cartridge type indicates the diameter of projectile and the length of the cartridge that holds it; for example 20×102 mm 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.