20mm

Twenty millimeters (20 mm) is a unit of length in the metric system equal to 0.02 meters, or about 0.787 inches. It is commonly used to express dimensions in science, engineering, and manufacturing, including bore diameters, fastener sizes, and tolerances.

In military terminology, 20 mm refers to a caliber—the diameter of the projectile. 20 mm ammunition has been used in autocannons and anti-aircraft guns, with various mounting configurations such as single-barrel, twin-barrel, and rotary designs. Weapons chambered for 20 mm rounds have been deployed by multiple countries since the early to mid-20th century and have been used against aircraft, light armor, and surface targets. The effectiveness of 20 mm systems depends on ammunition type, including high-explosive, armor-piercing, and tracer rounds, as well as the fire-control and platform employing the weapon.

Outside of weaponry, 20 mm appears in general engineering and manufacturing to specify dimensions, such as hole sizes, shafts, connectors, and other components. It is a common unit in mechanical drawings and product specifications, reflecting the broader adoption of the millimeter scale in international standards.

In summary, 20 mm denotes a size used across measurements and engineering, and, in a military context, designates a caliber for various autocannon and anti-aircraft systems.