Selective fire is the capability of a weapon to be adjusted to fire in semi-automatic, fully automatic, and/or burst mode. The modes are chosen by means of a selector switch, which varies depending on the weapon's design. Some selective-fire weapons have burst fire mechanisms to limit the maximum number of shots fired automatically in this mode. The most common limits are two or three rounds per trigger pull. Fully automatic fire refers to the ability for a weapon to fire continuously until either the feeding mechanism is emptied or the trigger is released. Semi-automatic refers to the ability to fire one round per trigger pull. The presence of selective fire modes on firearms permits more efficient use of rounds to be fired for specific needs, versus having a single mode of operation, such as fully automatic, thereby conserving ammunition while maximizing on-target accuracy and effectiveness. This capability is most commonly found on military weapons of the 20th and 21st centuries. History Early attempts at this technology were hindered by one or both of two obstacles: over-powerful ammunition and mechanical complexity. The latter led to excessive weight and unreliability in the firearm. One of the earliest designs dates to just before the end of the 19th century with the development of the Cei-Rigotti, an early automatic rifle created by Italian Army officer Amerigo Cei-Rigotti that had selective-fire capability (single shots or burst). Another is the M1918 Browning Automatic Rifle (BAR) developed during the First World War. The BAR and its subsequent designs incorporated a variety of selective-fire functions. The first design (M1918) is a selective-fire, air-cooled automatic rifle that used a trigger mechanism with a fire selector lever that enabled operating in either semi-automatic or fully automatic firing modes. The selector lever is located on the left side of the receiver and is simultaneously the manual safety (selector lever in the "S" position – weapon is "safe", "F" – "Fire", "A" – "Automatic" fire). The next version (M1918A1) had a unique rate-of-fire reducer mechanism purchased from FN Herstal with two rates of automatic fire. This reducer mechanism was later changed to one designed by the Springfield Armory. The final version (M1918A2) provided two selectable rates of fully automatic fire only. During World War II the Germans began development of the selective-fire function which resulted in the FG 42 battle rifle developed in 1942 at the request of the German Air Force