The breech mechanism (fig. 4-63) opens and closes the breech, extracts spent powder cases or guided

Figure 4-63.-Slide and slide-mounted components.

projectile (GP) shrouds from the breech, and ejects gas from the gun barrel. The main components are a breechblock, an empty case extractor, and a gas ejector.

General Operation

When the rammer transfers around from the cradle to the breech or completes the loading of a (GP) powder case behind a guided projectile (GP), the breechblock lowers to close the breech, to connect a firing cable, and to position a firing pin against the powder case primer.

When the round fires, recoil drives the gun barrel housing rearward, or out of battery position. The moving housing, which slides on horizontal keys in the slide, drives the recoil pistons against hydraulic fluid to absorb the shock and bring the recoil movement to a halt. Pressurized nitrogen then drives the counterrecoil pistons forward to put the gun barrel housing in battery position. The counterrecoil pistons also help buffer the shock of recoil.

Counterrecoil movement triggers the hydraulic actions that raise the breechblock. With the breechblock fully raised, two arms of the empty case extractor pull the spent case out of the breech and into the empty case tray. At the same time, the gas ejector releases pressurized air into the breech to clear the gun barrel of gases.

The empty case tray and the empty case ejector, mounted atop the slide, receive the spent powder case from the breech and eject it out of the gun shield and onto the weather deck. An ejector pawl moves the powder case from the empty case tray into an ejector tube, which leads to an opening in the gun shield.

The gun barrel, which consists of a tube and a liner, seats ammunition and, upon firing, imparts rotation to the projectile. Interrupted threads in the tube lock the barrel to the gun housing. Rifling in the liner imparts clockwise rotation to the projectile.