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 relatively few weapons (aside from shotguns and large game hunting rifles) which have been built which fire projectiles between .50 caliber (0.50 inch/12.7 mm, roughly 12 mm caliber) and 20 mm caliber, though the 14.5 mm caliber was used by some Soviet machineguns such as the KPV and antitank rifles such as PTRS and PTRD.

A very small number of anti-tank rifles have been produced in 20 mm and up calibers.

20 mm caliber cartridges have an outside shell diameter and inside barrel diameter of 0.8 inch (20 millimeters). Projectiles or shells are typically 75-100 mm (3-4 inches) long. Cartridges are typically 75-125 mm (3-5 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 53,600 joules (or 39,500 ft·lbf).

Usage

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.