ammunition, projectiles and propelling charges used in small arms, artillery, and other guns. Ammunition size is usually expressed in terms of calibre, which is the diameter of the projectile as measured in millimetres or inches. In general, projectiles less than 20 mm or .60 inch in diameter are classified as small-arm and larger calibres are considered artillery. A complete round of ammunition consists of all the components necessary for one firing of the gun. These normally include a projectile, the propellant, and a primer that ignites the propellant. Other components such as cartridge case, fuze, and bursting charge are frequently included.
In artillery ammunition, a fixed round is a complete round in which all components are securely joined by a cartridge case. (Though brass was used almost invariably for cartridge cases before World War II, it has since been largely superseded by steel.) In semifixed ammunition, the projectile is detachable from the cartridge case, an arrangement that allows for the size of the propelling charge to be adjusted, after which the projectile can be inserted loosely into the case. In separate-loading ammunition, a complete round consists of three components: the fuzed projectile, the propellant (in several combustible cloth bags), and the primer. This type of round is used in the largest-calibre guns because its separated components are easier to handle.