Caliber .50 AA Machine Gun

Chapter III

a. Development

Machine guns were adapted for anti-aircraft fire in order to augment the defensive power and characteristics of the 3" and 5" AA batteries by providing a higher rate of fire plus more rapid pointing. In their use emphasis was placed on defense against diving and other forms of close-in attack. The problem was a new one and until the declaration of the national emergency in the fell of 1939, progress was relatively slow. However, there were many factors contributing to this delay and in tracing the development it is interesting to note the complete reversal of opinion as to suitable weapons.

The caliber .50 machine gun was the first type extensively installed. To increase the volume of fire a twin mount was issued experimentally. It had conventional train, elevating and sight mechanism. The reception of this mount by the Fleet was entirely negative. The design was criticized as too slow in all operating functions, and there was a demand for a free swinging single gun mounting which would permit rapid, direct, universal motion, together with a simple fixed sight.

The next step was the single free swinging mount, the Mark 3. Elevating and training gear were eliminated, a universal movement



of gun was introduced, and the ring type sight was supplied. The reception of this mount was favorable and a manufacturing and installation program was undertaken to supply these mounts as rapidly as funds became available.

During this period two outstanding difficulties were encountered with the manner of controlling the fire of the guns--tracers vs. sights and the problem of obtaining a suitable target.

In firing against an airplane in a steep dive the position angle of the target was high and fairly constant and the trajectory comparatively flat. The first proposed solution of the control problem was tracer control, the sights being used to get on the target before opening fire. Tracer limitations were soon realized due to the limited visibility range, the phenomenon of tracer illusion, and the inability to judge the point at which the tracer massed the target. Sight control and a combination of sight and tracer control with fixed and movable sights were next introduced and for some time the problem was thrown from one to the other, or a combination of both.

The first effort toward supplying a target took the form of the aerological pilot balloon. With well-chosen wind, course and speed conditions the balloon provided a target at a fairly constant and high position angle, but it was going away from the ship and can not be recovered and examined for hits.


King Kites were resurrected for trial in the hope that hits could be counted. They were difficult to fly and could not be raised to suitable elevations and had to be discarded.

Missile targets in the form of weighted parachutes dropped from planes were proposed, but to obtain the required speed of descent the weights were considered too great a hazard to ship personnel and the target was discarded.

Next came the 10-foot helium balloon which was but an oversize modification of the original type, but with one of the disadvantages of the pilot balloon eliminated in that it could be recovered and the hits counted.

During the search for a suitable target what had happened to the .50 caliber machine gun and its mount? They too were experiencing difficulty. As a result of service firing, the ammunition, the mount, and the sights were criticized. It was thought that there was a marked difference in the trajectories of the tracer and the ball but investigation showed no significant deviation. Elimination of the traverse feature was tried without improvement. To reduce mount vibration, the supporting structure was stiffened, and anti-vibration pads between deck and mount were installed. While these experiments were in progress the perfect target appeared — the radio controlled drone. The first dive attack resulted in an



excellent performance for the .50 caliber and this was maintained in subsequent trials. As a result, the gun increased in favor with the Fleet and by 1938 was recognized as one of our primary weapons against dive bombers.

This acceptance, however, was short lived. Reports of naval operations in the European War, and the rapid increase in speed and armor of aircraft illustrated the relative weakness of the gun. By the stunner of 1940 it was apparent that the .50 caliber was not an effective weapon against diving or low flying planes. This conclusion was reached in the light of the fact that:

(1) A very large number of .50 caliber hits were required to give a reasonable probability of disabling the plane.

(2) Only a very small number of .50 caliber hits on any one diving airplane were attainable from any one gun.

(3) There were insufficient favorable gun positions to permit the installation of the numbers of .50 caliber mounts that would be required for a reasonably adequate defense.[1]

The gun selected to replace the .50 caliber was the 20mm Swiss Oerlikon--a gun which fired an explosive bullet about 2 1/2 times the weight of the .50 caliber at a rate of 450 rpm, and estimated at



the time to be 8 or 10 times as effective.[2] The Oerlikon had proved its worth in the Spanish Civil War, and in the European War which followed it soon became one of the primary AA weapons.

The decision to replace the .50 caliber was not followed by a cancellation of production contracts. It was realized that it would take some months to set up production lines for the 20mm, and in the meantime it was deemed unwise to discard the .50 caliber until production and installation of the new gun was well under way. Then too, for defense of small vessels where weight and space considerations were paramount the utility of the .50 caliber was well recognized.[3]

b. Procurement

Production of .50 caliber mounts by the Navy for the years 1940-1945 totaled 53,974. Guns for these mounts -- water cooled, aircraft basic, and heavy barrel air cooled -- were procured from the United States Army. In return the Navy supplied the Army with the Mark 17 Mod 1; Mark 1; Mark 26; and Mark 30 type mounts.




This joint procurement program worked smoothly and was described as entirely satisfactory by the cognizant Navy procurement section.[4]

At the time of Pearl Harbor there were 3,190 caliber .50 mounts in service or production. During the next two years the program was accelerated and 35,124 mounts, over 65% of the total procurement, came off the production lines. The peak period came in the first six months of 1942 when under the impact of war production requirements for the .50 caliber were advanced to supplement gradually increasing 20mm program.[5] This period of activity was followed by six months of relative quiet, and it was not until early 1943 that the program once again became active. The revival was due to the improvement in the design of several of the mounts plus an increasing demand for guns to meet the requirements of the expending small boat and landing craft programs.[6]

The Mark 17 mount, a scarff ring twin .50 caliber, was changed from a ball bearing to a roller bearing assembly, thus overcoming the breaking of bearings and the bending of scarff rings. The mount became standard equipment on motor torpedo boats, aircraft rescue boats, landing craft control boats, landing craft support



ships, harbor picket boats and motor gun boats.

The Mark 21 mount, originally designed for the British, was a heavy, hard to operate pedestal mount. In improving the design the mount was lightened 850 pounds, a feedway adapter was developed to take the Navy ammunition box Mark 6, and aircraft basic or heavy barreled guns were used in place of water-cooled guns. The mount had a small working circle and with the considerable weight reduction, became popular for use on landing craft mechanized (LCM2), YMS's, YT's, and LSM's.

The Navy had no light, efficient and easily broken down .50 caliber AA gun mount for advance bases. The mounts used were the old Army M-2, A-l, a heavy, unwieldy tripod mount and the Navy's Mark 24. To remedy this deficiency, the Mark 30 was developed. The mount came with removable tripod legs and also a pedestal base with quick-acting clamps. Thus it could be mounted aboard ship or used on shore with the tripod legs.

The popularity of the new mounts accounted for roughly 10,000 of the 16,397 production for 1943. Daring 1944 and 1945 the production program centered around the Mark 21. A new mount, the Mark 26, was developed for use on small craft such as PT's and mine sweepers. It was rigid without the shock absorbing cradle and could



be moved around on the ship. Procurement of the new mount totaled 1,942.

During 1945 thought was given to the development of a power driven quad mount for use against Japanese Kamikazes. Sixty M-45 quad mounts were procured from the Army and modified for shipboard use. They were power driven from their own power source, a gasoline engine and generator with batteries. The mount after modification was designated the Mark 32. It was tested at antiaircraft training centers and aboard several carriers but the consensus was that it was no better than the 20mm twin free swinging mount. The project was subsequently abandoned.[7]

While .50 caliber machine gun procurement was never more than a minor ordnance program it did produce two outstanding contractors—the Heintz Manufacturing Company of Philadelphia, Pennsylvania and the Continental Gin Company of Birmingham, Alabama. These two companies produced over 40,000 mounts or roughly 4/5 of the entire production, and it was largely through their excellent manufacturing abilities that there were no serious difficulties in procurement or deliveries. Other contractors included the Borg Warner Corporation of Muskegon, Michigan; the General Bronze Corporation, Long Island City, New York; the Atwood Machine Company, Stonington, Connecticut; the York Safe and Lock Company, York, Pennsylvania; and in the very early clays of the program, the Naval Gun Factory, Washington, D. C.



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Transcribed and formatted by Thomas Wildenberg, HyperWar Foundation