In automatic firearms, burst mode or burst fire is a firing mode enabling the shooter to fire a predetermined number of rounds, usually 2 or 3 rounds and 100+ on anti-aircraft weapons, with a single pull of the trigger. This firing mode is commonly used in submachine guns, assault rifles and carbines. Other types of firearms, such as machine pistols (e.g., the Beretta 93R) may also have a burst mode.
The burst mode is normally employed as an intermediate fire mode between semi-automatic and fully automatic, although some firearms lack a "full auto" capability and use a burst mode instead. For instance, the M16A2 (the standard-issue service rifle of the U.S. military) has, in addition to the semi-automatic mode, a 3-round burst mode, which replaced the fully automatic mode of the previous M16A1. The reason for this replacement was the massive waste of ammunition and very poor performance of soldiers who fired their rifles in fully automatic mode during the Vietnam War.
The number of rounds fired in a burst is almost universally determined by a cam mechanism that trips the trigger mechanism for each shot in the burst. Some designs, as employed on the M16A2, will terminate the burst if the trigger is released before the burst is complete, while others will reset the cam position, so the next burst will fire a full number of rounds.