Trigger crank

A trigger crank is a device that allows a semi-automatic firearm to fire at an increased rate. The trigger crank typically consists of a screw-tight clamp and crank assembly. The crank assembly is clamped onto the trigger guard of a semi-automatic firearm. The device is positioned in front of the trigger. When the crank is turned, tiny gears depress the trigger and cause the weapon to fire.

Internally, the firearm is not altered. As in all semi-automatic firearms, only one round is fired with every stroke of the trigger. This makes the "trigger crank" avoid classification as a machine gun within the definitions used by United States federal law, so as stated in an IRS revenue ruling and various other private-letter rulings by ATF. However, a battery-powered "trigger crank" (and by extension Gatling gun) is a machine gun as was determined by the ATF in 2004.

However, as with all private-letter rulings, this determination on the U.S. legality of trigger cranks is limited to the facts regarding the specific devices being examined. These opinions 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.

The devices have elicited scrutiny by gun control advocates and media commentators because of the perceived lax regulation placed upon them.