SIG MKMS - Delayed Blowback Action

Delayed Blowback Action

The locked-breach blowback system mirrored that of John Pedersen's Remington Model 51 only with a locking recess above rather than below the bolt. The longer barrel of the MKMS required either a bolt of greater mass or a locking system. When the firearm is in battery, the breech block rests slightly forward of the locking shoulder in the frame. When the cartridge is fired, the breech block and bolt carrier move together a short distance rearward powered by the energy of the cartridge as in a standard blowback system. When the breech block contacts the locking shoulder, it stops, locking the breech. The bolt carrier continues rearward with the momentum it acquired in the initial phase. This delay allows chamber pressure to drop to safe levels while the breech is locked and the cartridge slightly extracted. Once the bullet leaves the barrel and pressure drops, the continuing motion of the bolt carrier cams the breech block from its locking recess, continuing the operating cycle.