The first "rapid firing" firearms were usually similar to the 19th century Gatling gun, which would fire cartridges from a magazine as fast as and as long as the operator turned a crank. Eventually, the "rapid" firing mechanism was perfected and miniaturized to the extent that either the recoil of the firearm or the gas pressure from firing could be used to operate it, thus the operator needed only to pull a trigger (which made the firing mechanisms truly "automatic"). An automatic (or "fully automatic") firearm is one that automatically re-cocks, reloads, and fires as long as the trigger is depressed. An automatic firearm is capable of firing multiple rounds with one pull of the trigger. The Gatling gun may have been the first automatic weapon, though the modern trigger-actuated machine gun was not widely introduced until the First World War with the German "Spandau" and British Lewis Gun. Automatic rifles such as the Browning Automatic Rifle were in common use by the military during the early part of the 20th century, and automatic rifles that fired handgun rounds, known as submachine guns, also appeared in this time. Many modern military firearms have a selective fire option, which is a mechanical switch that allows the firearm be fired either in the semi-automatic or fully automatic mode. In the current M16A2 and M16A4 variants of the U.S.-made M16, continuous fully automatic fire is not possible, having been replaced by an automatic burst of three cartridges (this conserves ammunition and increases controllability). Automatic weapons are largely restricted to military and paramilitary organizations, though many automatic designs are infamous for their use by civilians.