a bump stock utilizes the natural recoil motion of a semi-automatic rifle to facilitate rapid fire similar to an automatic firearm without requiring any modification of internal mechanisms to convert the firearm to an automatic weapon. it consists of a sliding stock with an attached support step in front of the trigger. pushing forward on the barrel, or forward grip, causes the gun to slide forward in the stock, "bumping" the trigger against the trigger finger that rests on the support step, firing the gun. the gun then recoils, moving the trigger backward, away from the finger. continued forward pressure on the barrel or forward grip returns the trigger to the finger, repeating the cycle. the amount of forward pressure controls the rate of fire, which can reach several hundred rounds per minute.