A machine gun is a fully automatic mounted or portable firearm designed to fire bullets in rapid succession from an ammunition belt or magazine, typically at a rate of 300 rounds per minute or higher. Not all fully automatic firearms are machine guns. Submachine guns, rifles, assault rifles, battle rifles, shotguns, pistols or cannons may be capable of fully automatic fire, but are not designed for sustained fire. As a class of military rapid-fire guns, machine guns are fully automatic weapons designed to be used as support weapons and generally used when attached to a mount- or fired from the ground on a bipod or tripod. Many machine guns also use belt feeding and open bolt operation, features not normally found on rifles.