A hell-fire trigger is a device that allows a semi-automatic firearm to fire at an increased rate.
This makes the "hell-fire trigger" 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.
[1] 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.
During the Waco siege, David Koresh, leader of the Branch Davidians, reportedly told authorities that he utilized semi-automatic guns with the part installed.