lockedblowback

Locked-blowback is an action used in some semi-automatic firearms in which the breech is mechanically locked at the moment of firing to contain the high pressure in the chamber, after which it unlocks to allow the slide or bolt to move rearward. It is often described as a form of delayed blowback and sits conceptually between simple blowback and locked-breech recoil systems.

In practice, a locking mechanism such as a locking block, a rotating locking piece, or a tilting lock keeps the breech closed for the initial portion of the recoil. The barrel and slide then travel together briefly, then unlock as pressure declines, and the slide continues rearward to extract and eject the spent case. Different locking geometries yield varying timing and travel, but the core idea is to delay opening of the breech just long enough to manage chamber pressure.

Variants use different locking methods, including tilting blocks, rotating blocks, or lugs that engage recesses. These designs aim to stabilize chamber pressure without the heavier recoil system of a true locked-breech or the simpler mass of a pure blowback. They are most commonly associated with compact or moderate-caliber firearms but can be applied to other configurations as well.

Advantages of locked-blowback include enabling higher-pressure cartridges to be used in blowback-like layouts and potentially avoiding a heavy recoil spring or mass. Disadvantages include added mechanical complexity, potential reliability concerns, and typically slower cycling than some recoil-operated designs.

Locked-blowback is related to but distinct from gas-delayed and roller-delayed blowback. In modern firearms practice, it remains relatively uncommon compared with other action types, with most designs preferring simple blowback for small calibers or short-recoil systems for larger ones.