A selective fire firearm has at least one semi-automatic and one automatic mode, which is activated by means of a selector which varies depending on the weapon's design. Some selective fire weapons utilize 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 rifle to fire continuously until the magazine is empty. "Burst-capable" fire refers to the ability of a rifle to fire a small, fixed number of rounds (usually two or three) with one trigger pull. Semi-automatic refers to the ability to fire one round per trigger pull. The presence of selective fire modes on assault rifles 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.
Selective fire weapons, by definition, have a semi-automatic mode, where the weapon automatically reloads the chamber after each fired round, but requires the trigger be released and pulled again before firing the next round. This allows for rapid and (in theory) aimed fire. In some weapons, the selection is between different rates of automatic fire and/or varying burst limiters. The selection is often by a small rotating switch often integrated with the safety catch or a switch separate from the safety, as in the British SA80 family. Another method is a weighted trigger, such as the Steyr AUG, which will fire a single shot when 8.8 – 15.4 pounds of weight is exerted on the trigger, and then become fully automatic when over 15.4 pounds of weight is applied. This is useful for emergency situations where a rapid volley of rounds is more effective for suppressing a close enemy rather than a single round burst.
Some selective fire weapons offer a burst mode as the second option, where each pull of the trigger automatically fires a predetermined number of rounds (generally two or three), but won't fire any more until the trigger is pulled again. The current U.S. standard assault rifle, the M16A4, and the M4 carbine variant of this rifle fire a maximum of three rounds with each pull of the trigger in burst mode. In this design, it retains the count of previously fired rounds and may fire fewer than three rounds. Other designs reset the count with each trigger pull allowing a uniform three round burst as long as rounds remain.
A common version of the Heckler & Koch MP5 submachine gun (widely used by SWAT teams and military special operations personnel) fires single shots, three-round-bursts, and automatically. A special variant uses a two-round-burst to minimize the chances of missing with a third round. Some automatic cannons have larger burst limiters to coincide with higher rates of fire.
Selective fire weapons are regulated in the United States under the National Firearms Act of 1934; their importation was prohibited in 1968; their new manufacture for the civilian market was prohibited by the Firearm Owners Protection Act of 1986. Those still in circulation often command prices significantly higher than similar models still available to law enforcement and military agencies.