Hell-Fire trigger

A Hell-Fire trigger is a device that allows a semi-automatic firearm to fire at a rate approaching that of a fully automatic firearm. 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 reducing the amount of finger movement required to actuate the trigger and allows for a significantly higher rate of fire. 
Technically, the firearm is not altered. As in all semi-automatic firearms, only one round is fired with every stroke of the trigger. 
The Hell-Fire trigger was used, with a pair of TEC-9 pistols, by the perpetrator of a 1993 mass shooting that killed nine people. Separately, David Koresh, leader of the Branch Davidians, reportedly told authorities that he utilized semi-automatic guns with the part installed.