bump fire

In firearms, bump fire is the act of using the recoil of a semi-automatic firearm to fire shots in rapid succession, which simulates the ability of a fully automatic weapon.

The bump firing process involves bracing the rifle with the non-trigger hand, releasing the grip on the firing hand (leaving the trigger finger in its normal position in front of the trigger), pushing the rifle forward in order to apply pressure on the trigger from the finger, and keeping the trigger finger stationary. During a shot, the firearm will recoil ("bump" back) and the trigger will reset as it normally does; then the non-trigger hand pulls the firearm away from the body and back to the original position, pressing the trigger against the stationary finger again, thereby firing another round when the trigger is pushed back.
Normally, a rifle is held securely and firmly against the shoulder but the loose shoulder hold that creates the rattle to rapidly depress the trigger affects accuracy in a way that is not encountered with firearms that are designed for select-fire.
A similar method can be employed with semi-automatic pistols, in which one hand holds the grip, two fingers are placed in the trigger well, and then the grip hand shoves the weapon forwards while the trigger fingers remain stationary. With revolvers, rapid fire can be achieved by using two trigger fingers firing offset.
All these techniques greatly degrade the accuracy of the firearm. The techniques trade accurate, aimed fire for an increase in the firearm's rate of fire. The inaccuracy renders the practice uncommon. None of these techniques fire more than one round with a single trigger pull; rather they compensate for biomechanical limitations associated with how fast a finger can repetitively pull the trigger.
With bump-firing it is common to use all the rounds in the weapon's magazine, but it becomes easy to create a stoppage as the cycling of all semiautomatic firearms requires the bolt to move against the stationary firearm (low-mass semiautomatic pistols suffer from the same problem due to "limp wristing"). The bolt must complete the stroke against the spring and that doesn't happen if both the bolt and spring are moving rearward. Non-bump fire rifles can suffer the same failure from fouling or for undercharged ammunition. Blanks also can cause these failures. Additionally, it is possible that if a weapon is bump-fired too fast, the hammer will be released before the bolt closes. This will either cause the hammer to "ride" the bolt carrier without firing the chambered round, or cause the weapon to slamfire. If a weapon with a locking bolt slamfires, there is an inherent danger of the weapon firing before the bolt is fully closed and locked, potentially causing an explosion and injuring the user.

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