M3 .50 Caliber Machine Gun

The M3 .50 caliber machine gun (also referred to as the AN/M3 .50 caliber machine gun) is an automatic, recoil-operated, link-belt-fed, air-cooled machine gun having a rate of fire over 1,000 rounds per minute. A metallic link disintegrating belt is used to hold the ammunition while it is being fed into the gun. By means of changes of position for some of the components, the gun may setup so that the ammunition can be fed from either the left- or right-hand side. Guns can be configured to be able to feed only from one direction or be able to be fed from both directions, albeit not simultaneously.

The gun can be mounted in the wing or fuselage of a fighter plane. It may also be mounted in a turret of a bomber. It can also be mounted in a detachable gun pod for use on fixed and rotary wing aircraft. The gun fires percussion type primed ammurution. A solenoid, which can be mounted on the top plate or on either side of the gun, depending upon what side the sear slide is assembled, is used to fire the gun.

The M3 .50 caliber machine gun is visually similar to the M2 .50 caliber machine gun, but features numerous important differences in most of the internal components. The recoil booster, cover group, belt holding pawls, breech lock, barrel, and short round stop are used in both the M3 and M2 machine guns.

As early as the 1930s, there had been a desire with the US military for a variant of the M2 .50 caliber machine gun with a dramatically higher rate of fire for use in aircraft. During a conference held at the Aberdeen Proving Ground on 18 June 1937, a group of Air Force personnel, headed by Gen. H. H. Arnold, requested that the Ordnance Corps undertake to increase the rate of fire of the M2 aircraft gun, which was then recorded as 600 rounds per minute for a 750-grain projectile fired at a velocity of 2,500 feet per second.

Late in September 1939, a formal statement of the military characteristics desired in an improved caliber .50 machine gun was made by the Chief of the Army Air Corps. The first requirement was for a weapon with a maximum cyclic rate consistent with other design requirements, not less than 1,000 rounds per minute. Major consideration in the development of subsequent weapons to fit this requirement was given to this feature. The other 11 requirements included stipulations on dimensions (to allow it be mounted in fixed and flexible roles in aircraft), weight, and effectiveness (required penetration of 0.75 inch armor plate at 600 yards with armor piercing rounds).

Following the conference at Aberdeen in 1937, Springfield Armory and the Colt plant had both begun work on a high rate of fire variant. This work continued through the establishment of formal requirements in 1939. It was not until January 1940, however, that a high cyclic rate gun was submitted for test at Aberdeen. This weapon, produced by Colt in cooperation with Springfield Armory, fired at a rate of 997 rounds per minute, but was deemed unsatisfactory because of excessive breakages and malfunctions. Continued work was done to improve the design, eventually designated as the T21, with unfavorable results.

During the early part of 1942, the High Standard manufacturing Company designed another high cyclic rate M2 variant made 2 prototype models. These guns, designated T22, were submitted to the Aberdeen Proving Ground for test on 10 August 1942. One gun was fired on that date, 551 rounds at a cyclic rate of 1,066 rounds per minute, during which 5 stubbed rounds and 2 failures to feed occurred. Both guns were returned to High Standard for modification. The T22, however, showed more promise than the T21, and further development was conducted. A total of 7 subvariants were produced between August 1942 and December 1943, all of which continued to suffer from excessive wear, and subsequently breakages and malfunctions, associated with the high rate of fire.

So that no practicable approach to the development of an acceptable high speed gun might be overlooked, a contract was placed on 4 August 1943 with the Frigidaire Division of the General Motors Corporation, to cover the development of what would be known as the T25 series. The basic mechanism of the of the M2 gun was to he used in this development, but no restrictions were place on the number of changes that could be made to the M2 design gun and no requirement for interchangeability of the components of the 2 guns was imposed.

The development was to begin by correcting certain known mechanical weaknesses of the M2 gun, and to proceed by making all changes necessary to provide reliable functioning at a rate of fire of 1,200 rounds per minute. For example, the back plate buffer, barrel buffer, and receiver were all redesigned.

By 1944, the Navy Bureau of Ordnance had become interested in the development and also supported continued work on what was then the most up to date version of the T22, the T22E6. Replacement parts that could be applied as a kit to existing M2 aircraft guns to provide the higher rate of fire were proposed and the design became known as the T27. The first 2 models of the T27 were submitted to Aberdeen for test on 1 January 1944. The test of both guns were discontinued before the completion of the regular 5,000 round test. In the 14 months following the delivery of the first 2 T27s, 18 more guns in at least 8 total configurations were tested at Aberdeen. The results showed excessive malfunctions and breakages, representing actual danger to personnel and equipment.

The kit that was provided for the conversion of M2 aircraft guns to the T27E7 configuration had a total of 7 components: A bolt stabilizer, to stabilize the bolt and depress the extractor at the end of recoil; muzzle booster, to increase cyclic rate; side plate switch, strong spring, and changed contour; special extractor without ejector; special bolt with spring clips to position rounds in the T-slot, in lieu of the ejector; split holding prawls to improve feeding; and a rubber plug for the back plate buffer, in lieu of the existing fiber discs. The oil in the buffer was also drained.

It was not until 10 March 1941 that the first of the T25 guns was submitted to Aberdeen Proving Ground for test. The gun gave a fair performance for 2,000 rounds. After that, excessive breakages and malfunctions occurred, and at 3,100 rounds the test was discontinued because of the breakage of the back plate buffer. Work was immediately started on a second gun, designated the T25E1. This gun was tested at Aberdeen on 10 May 1944. This gun was completely unsatisfactory. The bolt, top cover, and recoil booster of the T22E6 was added to the T25E1, resulting in the T25E2, tested at Aberdeen on 1 June 1944. This weapon too was deemed unsatisfactory.

The nested helical springs used in the variants of the T25 were determined to be the source of a number of issues and were placed Belleville spring washers on a new variant, the T25E3. The first these guns was tested at Aberdeen on 19 July 1944. The functioning was greatly improved and the weapon achieved a cyclic rate of fire of 1,250 rounds per minute. Continued testing, by both the Army Air Forces and the Navy of the T25E3 continued through 1944. The weapon was standardized as the M3 in April 1945, with the M2 aircraft gun being immediately reclassified as limited standard. Minor modifications were made after acceptance to further improve the weapon's reliability, all of which were incorporated into the basic M3 design.

The M3 .50 caliber machine gun quickly became a standard aircraft armament for a number of fighters and bombers used by the US Army Air Forces and then the fledgling US Air Force, as well as the US Navy and Marine Corps. A gun pod was developed to utilize the weapon on both fixed and rotary wing aircraft. The M3 was eventually supplanted on fixed wing aircraft by various types of 20mm and 30mm cannon. Derivatives of the weapon, however, were continued to be used as helicopter armament, in both fixed and flexible installations. A derivative of the weapon was also used as a secondary weapon on the US Army's Avenger air defense system.

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