A term used to describe a method of locking found most often in gas operated firearms in which the bolt, upon contact with the breech rotates and locks into place, being held in place by lugs attached to the breech or barrel extension. The gas-operated system for implementing automatic reloading of a firearm is one of five such systems, the others being recoil-operated, gatling, chain, and blowback. ...

Image File history File links Bolt and barrel extension from M16 rifle. ...

M16 bolt (left) and barrel extension (right). Upon closing, the bolt passes through the slots cut in the front of the barrel extension, and then rotates slightly (~4mm) to the left at which point it is locked in place until the action is cycled, either manually by the operator, or mechanically by the gas created by discharging the rifle pushing on the operating rod or bolt carrier, which then rotates the bolt to the right and unlocks it from the breech.