The breech mechanism is a downward sliding semi-automatic breechblock. Its breech uses a horizontally sliding breechblock for loading the fixed cartridge cases. The underside of the breechblock is slotted to travel over the ejector. In toggle-delayed blowback firearms, the rearward motion of the breechblock must overcome significant mechanical leverage. This type of breechblock can be adapted to cycle by lever, cocking handle or gas. It was equipped with a horizontal sliding block breechblock, hydraulic recoil buffer and spring recuperator. The VB, like the Bren light machine gun, used a locking tilting breechblock. The T254E2/M68 used a vertical sliding breechblock instead of the T254E1's horizontal breechblock. It used a vertical sliding breechblock instead of the T254E1's horizontal breechblock. The AM-23 aircraft cannon is a gas-operated weapon with a vertically moving wedge breechblock. The A-12.7 machine gun is a gas-operated weapon with a vertically moving wedge breechblock. The firing pin was housed within the breechblock. Firing force is contained by the rear of the breechblock bearing on the receiver. It had an interrupted thread type, drop breechblock mechanism. The action utilizes a toggle-joint system to lock the breechblock.