CHAPTER-9-C

NAVAL ORDNANCE AND GUNNERY, VOLUME 1

CHAPTER 9
AUTOMATIC WEAPONS

HOME INDEX
Chapter 9 Automatic Weapons
A. Introduction
B. 20-mm aircraft gun
C. 40-mm guns and mounts
D. 3"/50 rapid-fire guns and mounts

C. 40-mm Guns and Mounts

9C1. General

The 40-mm gun is a recoil-operated, heavy machine gun designed primarily for AA fire. Its distinctive features include (1) a vertical sliding-wedge breech mechanism, (2) a hand-fed automatic loader, (3) a spring-operated rammer, and (4) a trigger mechanism that controls the rammer operation only.

Once put in operation and the ramming cycle started, this gun loads and fires without further attention. It can be operated in either fully automatic or single fire. The maximum cyclic rate of automatic fire is about 160 rounds per minute.

For naval use, these guns are usually water-cooled and assembled in pairs, 1 pair making up a twin mount, 2 pairs a quad mount. The individual gun mechanisms are alike except for the changes necessary to make them right and left guns. Both twin and quad mounts are used on destroyer escorts, destroyers, and many classes of larger naval ships. Air-cooled single guns are used on some small craft.

The conventional mounts for twins and quads have power-operated elevating and training gear to position the guns as a unit. Power drives may be controlled at the mount by the pointer and trainer, or from the director through an electrical control system. The mounts can also be trained and the guns elevated manually by handwheels.

Ammunition is of the fixed type, loaded into clips containing four rounds each. The usual AA projectile is provided with a tracer and a nose-type impact fuze armed by the rotation of the projectile in flight. A complete round weighs about 5 pounds, the projectile about 2. The tracer is effective for from 7 1/2 to 8 seconds, after which it destroys the projectile by detonating the burster charge. Non-self-destructive AA and AP projectiles are also available. The service charge produces an initial velocity of about 2,800 feet per second. The maximum horizontal range obtained before self-destructive action, is about 5,000 yards; without self-destructive action, the range is increased to about 11,000 yards.

See figure 9C1 for the general arrangement of 40-mm quad assemblies.

The gun mechanism consists of the following five major components:

1. Barrel assembly. This consists of the gun barrel, its water jacket, the recoil spring, and the flash hider. The barrel is a single-piece forging 56 calibers long. It is rifled with 16 grooves having a right-hand increasing twist of 1 turn in 45 calibers at the origin to 1 turn in 30 calibers at the muzzle. The barrel passes through the cylindrical fore part of the slide and is attached to the housing within by means of a bayonet-type joint.

2. Slide, assembly. This has a box-shaped rear section and a cylindrical forward section, which serve to support and provide working surfaces for the other parts of the gun mechanism.

3. Breech-mechanism assembly. This consists of a housing assembly, a breechblock assembly, and associated operating parts. It is a recoiling part of the gun mechanism.

4. Loader assembly. The loader feeds cartridges to the rammer tray and catapults them into the firing chamber. The main operating parts are the loader, rammer tray, and rammer.

5. Recoil-counterrecoil system. Each gun has its own recoil-counterrecoil assembly, which consists of a recoil spring fitted over the after end of the barrel assembly and a hydraulic recoil unit secured to the under-side of the slide, with the piston rod attached to the housing. The spring is compressed during recoil and returns the gun to battery as the recoil piston moves back and forth in the hydraulic cylinder. At the same time a fluid mixture of glycerin and water is forced through a series of orifices. Both recoil braking and counterrecoil buffing are accomplished in this single cylinder unit. A needle valve adjusts orifice size, and its setting determines the time required for counterrecoil. The length of recoil is governed by the quantity, specific gravity, and temperature of the recoil fluid, and to a slight extent by the elevation of the gun. The setting of the needle valve in the recoil cylinder has no appreciable effect on the length of recoil, but does control the velocity of counterrecoil and, as a result, the rate of fire.

9C2. Mount

The 40-mm twin and quad gun assemblies have open base-ring mounts. The training circle is secured to the stand, and the carriage is supported on radial and thrust roller bearings. A platform on the after end of the carriage is used by the loaders when the gun is in operation. The cooling-system tanks and electrically driven circulating pumps are mounted on the after end of the platform. The firing mechanism is mounted on the forward face of the carriage. The slide trunnions are supported on roller bearings.

Individual power drives, electric-hydraulic or amplidyne, are used for the elevating and training gear. See chapter 10. With late models of twin mounts and with all quads, in local power control the pointer controls both train and elevation. With the electric-hydraulic system, he uses a single control lever or “joy stick”; with the amplidyne system as installed on quad mounts, he uses a pair of handles on a control box. For manual control, the pointer’s and trainer’s handwheels are geared directly to the elevating and training racks.

The mount can be trained 360° in either direction from the locked position. A training stop is provided to prevent training beyond these limits. A power-drive cut-off switch operates, when the limit is approached, to shut off the driving motor. The elevation limits, unless restricted for specific shipboard installations, are 15° depression and 90° elevation. Elevation and train centering pins lock the gun and mount in place when the gun is secured.

Open peep and ring sights are provided for the pointer and trainer. On some mounts, especially singles, lead-computing sights are also provided; but the normal method of control is by director.

9C3. Personnel

The basic gun crew for the twin or quad mount includes the following: a mount captain, a pointer, a trainer, 1 loader for each gun, and at least 1 ammunition passer for each gun. More handlers will be needed if the ready ammunition is stored at some distance from the mount.

The mount captain is in charge of the gun and crew. In local control, he designates targets and gives orders for firing. In addition to his regular duty of training the mount, the trainer releases the train centering pin and starts the circulating pumps in the cooling system. The pointer releases the elevation centering pin, starts the firing motor, elevates the gun with the handwheels, and operates the foot firing pedal. (Joy-stick control in both elevation and train may be given to the pointer when desirable.) The loaders set the firing-selector lever at the desired position (STOP FIRE, AUTO, or SINGLE), Operate the hand operating lever, and place the ammunition in the loader. The ammunition passers bring the clipped cartridges from the ready stowage or magazines and hand them to the loaders on the gun platform.