LEARNING OBJECTIVE: Describe the firing equipment common to all naval gun mounts.

The firing equipment includes all the components necessary to allow the gun to fire safely, absorb the shock of recoil, and reposition for further firing. This includes the housing, the breechblock, the recoil system, the counterrecoil system, the firing circuits, and the firing cutouts.

Housing-The housing is a large steel casement in which the barrel and breechblock are fitted. The housing moves in recoil inside the slide.

Breechblock-The breechblock seals the breech end of the barrel. Breechblocks are of two different types-sliding wedge or interrupted thread. The slidlng wedge consists of a machined steel plug that slides in a grooved way in the housing to cover the breech opening. The grooves are slanted so that the breechblock moves forward as it covers the back of the casing, wedging it in place. The interrupted thread breech plug closes similar to the way a bayonet lug camera lens is installed. The lugs on the plug are cut to allow it to be inserted into the grooves cut in the threaded breech. Once inserted, the plug is turned 120" and locked in place. The Mk 45 and Mk 75 use the sliding wedge breechblock. The sliding wedge breechblock is shown in figure 6-9.

Figure 6-9.-The sliding wedge breechblock.

Figure 6-10.-Recoil and counterrecoil systems.

Recoil System-Normally, a recoil system (fig. 6-10) consists of two stationary pistons attached to the slide, set in a liquid-filled cylinder in the housing. As the housing moves rearward in recoil, the trapped liquid is forced around the piston head through metered orifices, slowing the movement of the housing.

Counterrecoil System-A counterrecoil system consists of a piston (or pistons) set in a pressurized cylinder. As the gun recoils, the piston protrudes further into the cylinder. After the force of recoil is spent, the nitrogen pressure, acting against the piston, pushes the housing back into battery (the full forward position). The piston may be attached to the slide or set in a chamber mounted to the inside of the slide. Figure 6-11 shows the configuration used on the 5"/54 Mk 45 gun system. Nitrogen pressure holds the free-floating pistons against the back of the housing, which forces them into the stationary chamber during recoil.

Since the nitrogen pressure in the counterrecoil system is the only thing holding the gun in battery, all guns are equipped with a safety link. The safety link physically attaches the housing to the slide to prevent it from moving if system pressure is lost. The safety link is disconnected before firing.

Firing Circuits-Basically, a firing circuit supplies firing voltage to the propelling charge primer. This sounds simple, but the application can be quite complicated. Certain conditions must exist before

Figure 6-11.-5"/54 Mk 45 recoil and counterrecoil systems.

firing to ensure a safe evolution. Making sure the gun is pointed in a safe direction, all the loading equipment is in the FIRE position (out of the way of recoiling parts) and the breechblock is all the way closed are just a few of the obvious things that must be correct before firing. A typical electronic firing circuit includes interlock inputs that serve to monitor these and many other conditions, allowing firing voltage to pass only after all safety conditions have been satisfied.

Firing Cutouts-A firing cutout mechanism interrupts firing when the gun is pointed at or near permanent ship's structure. A firing cutout is a mechanical device that monitors the gun position. Figure 6-12 shows a firing cutout mechanism. Notice the inputs from the system. The gun train position input rotates the cam, while the elevation input positions the follower. While the cam follower is on a low area of the cam, the firing circuit is closed or enabled. As the gun trains and elevates and the follower rides upon a raised portion of the cam, the firing circuit is opened.