A trigger crank is a device that allows a semi-automatic firearm to fire at an increased rate.
The crank assembly is clamped onto the trigger guard of a semi-automatic firearm.
When the crank is turned, tiny gears depress the trigger and cause the weapon to fire.
This makes the "trigger crank" avoid classification as a machine gun for purposes of gun law in the United States, as stated in an IRS revenue ruling[1] and various other private-letter rulings by ATF.
[4] The devices have elicited scrutiny by gun control advocates and media commentators because of the perceived lax regulation placed upon them.