bump stock

A bump stock is a rifle stock designed to cause a semi-automatic firearm to move rearward and forward with each shot, causing the trigger to “bump” into the shooter’s trigger finger, held stationary, to “bump fire” the weapon in a more or less steady repetition. This thus enables a semi-automatic firearm to be used similarly to a fully-automatic firearm.

Such a device exists only to get around the federal restriction on fully-automatic firearms.