Hell-fire trigger

A hell-fire trigger is a device that allows a semi-automatic firearm to fire at an increased rate. The hell-fire clamps to the trigger guard behind the trigger and presses a "finger" against the back of the trigger to increase the force that returns the trigger to its forward position, effectively decreasing the time required for the trigger to reset, allowing for a faster follow-up shot.
Internally, the firearm is not altered. As in all semi-automatic firearms, only one round is fired with every stroke of the trigger. This allows the "hell-fire trigger" to avoid classification as a machine gun within the definitions used by United States federal law, as stated in an ATF private-letter ruling from 1990.
However, as with all private-letter rulings, this determination on the U.S. legality of hell-fire triggers is limited to the facts regarding the specific device being examined. The 1990 opinion may be modified or revoked at any subsequent time by the Bureau of Alcohol, [Tobacco, Firearms and Explosives]. Furthermore, agency opinion is not always considered legally binding.
During the Waco siege, David Koresh, leader of the Branch Davidians, reportedly told authorities that he utilized semi-automatic guns with the part installed. Another well-known case of its reported use is the 101 [California Street shooting].