A semi-automatic firearm whose breechblock and barrel are not mechanically locked together when fired. In such case the breechblock immediately begins to separate from the barrel upon firing. Blowback is used in comparatively low powered weapons, in which inertia of the breechblock, and cartridge wall adhesion against the chamber, are sufficient enough to retard opening until breech gas pressures have fallen to a safe level.
A type of action in an autoloading firearm where the breech is not locked. Rather, the recoil of the firing cartridge overcomes the inertia of a spring-loaded breechblock, forcing it back to cycle the action. A simple design, but limited to relatively low powered firearms---typically pistols of calibre .380ACP or less---or an impractically heavy breechblock would be required.