The 20 mm Caliber is a specific size of cannon or autocannon ammunition. There are few weapons which have been
The M61 Vulcan rotary cannon, a gatling gun that uses 20 mm ammuntion.
built that fire projectiles between .50 caliber and 20 mm caliber. 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 20 mm. Projectiles or shells are typically 75–127 mm long. Cartridges are typically 75–152 mm long. Many but not all 20 mm shells have an explosive filling and detonating fuze.