The 20 mm is a specific size of cannon ammunition.

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). Order 20mm Cannon Products