A Hellfire trigger is a device that allows a semi-automatic firearm to fire at a rate approaching that of fully automatic firearms. When the trigger is pressed, the Hellfire device disengages the trigger return spring, which moves the operator's trigger finger. As a result, the operator can move his finger to fire individual shots more quickly. Some users may use the Hellfire device in conjunction with bump firing techniques to fire more rapidly.
The device is simple, but unsafe and inefficient. Many users complain about the awkward firing position required to shoot a firearm equipped with a Hellfire trigger. Use of the trigger may damage a gun or cause it to malfunction, since most semi-automatic firearms are not designed to handle the stress of rapid fire.
While the company that makes the Hellfire sells each unit with a copy of the letter from US BATF stating that it is legal to own, many believe the nature of the device could conceivably cause legal problems for the owner through the appearance of having an illegally modified firearm. They cite the fact the warrant against David Koresh before the Waco Siege stated that he might possess rifles illegally modified to fire full auto, based on neighbors reports of hearing what sounded to be full auto fire coming from the area.
It is alleged that David Koresh actually owned a Hellfire device leading many to speculate that the sounds the neighbors heard were actually as a result of the (completely legal) Hellfire device rather than full auto modifications. Whether or not this is the case, the instructions from the manufacturer explicitly admonish the owner from being too overt in the use of the device for fear of any false accusations.