By Frank Iannamico
The early U.S. military priorities for a large caliber .50 machine gun were for the development of an aircraft gun, an infantry ground gun and lastly an auxiliary tank weapon: in that order. Although the caliber .50 Browning eventually became quite successful, it had humble beginnings. As an infantry weapon it was considered too difficult to control and as an aircraft weapon it fired too slow and was excessively heavy. A major problem was that the Army’s aircraft during the post World War I era were lightly constructed, and not well suited to handle the weight and recoil of the powerful caliber .50 Browning.
In the years following World War I, the .50 Browning machine gun slowly began to surpass the smaller caliber .30 guns in military aircraft applications. The proposed change from the .30 to the .50 Browning initially met resistance by Army Air Service pilots who preferred the much faster firing .30 caliber guns when aircraft were not armor protected prior to World War II. Despite the opposition from the pilots, the trend toward the .50 caliber gun continued, although the change over was quite gradual occurring over a period of years. The .30 caliber M2 Browning aircraft gun was capable of firing 1,200 rounds per minute. However, the .30 caliber round was simply not effective enough to successfully engage, and defeat the latest aircraft then being built. The .30 caliber’s muzzle velocity was low, and its projectile too light. During World War II, the U.S. Navy and Marine aviators who encountered the lightly constructed Japanese aircraft in the Pacific Theater relied on the .30 caliber guns for a longer period than their Army Air Corps counterparts in Europe.
In the years following World War I, the U.S. Air Corps had anticipated that with the heavy .50 caliber projectile and the uzzle velocity of the round, it could readily be developed into an effective armor- piercing, incendiary and even an explosive type cartridge. Such rounds could provide an effective means of shooting down enemy aircraft that were being built with an increasing use of armor protection. The time of projectile flight for the .50 caliber round was 0.7 seconds to travel 600 yards, and at that range penetration of 0.75 inch armor plate was possible when armor-piercing projectiles were employed.
The large and heavy .50 caliber weapons were originally contemplated for use on the wings, fuselage and turrets of low altitude bombers, medium pursuit and similar aircraft that generally operated at altitudes below 15,000 feet. Such aircraft could efficiently carry the heavy weapons and attending ammunition loads. Lighter high-altitude scout and reconnaissance aircraft could continue to be armed with the far lighter .30 caliber machine guns.
Although there were a few aircraft .50 caliber machine guns procured between the wars, the peacetime budget keep the numbers small. The early .50 caliber aircraft Brownings were primarily used to design and test suitable aircraft mounts and develop tactics for the gun’s use. The earliest .50s only fed from one side, complicating cramped aircraft installations
In May of 1923, the first air-cooled Model 1921 .50 caliber aircraft gun was standardized in both flexible and fixed configurations. The weapon was adapted from the water-cooled Model 1921 anti-aircraft gun. The weapon only fed from the left side of the receiver making multiple installations in some aircraft difficult. Further development of the .50 caliber aircraft gun resulted in the remaining Model 1921 aircraft guns to be declared obsolete on 26 March 1943. The model 1922 was very similar to the model of 1921 aircraft, except it was designed to be fed from either the left or right side of the receiver. A few additional specially designed parts were required to accomplish the conversion. The Model 1923 aircraft gun was designed for right or left hand feed by the repositioning of several components and, unlike the model of 1922, no additional parts were needed. This weapon was eventually standardized on 5 October 1933 as the M2, caliber .50 aircraft gun. The M2 could be installed on either a rigid, turret or hand operated mount. The weapon could be adapted to any existing aircraft by adding the necessary parts and accessories. This weapon could be fired either by a manual trigger, a mechanical or electrical device.
Just prior to the United States’ entry into World War II, the Colt’s Patent Fire Arms Company of Hartford, Connecticut was contracted by the Ordnance Department to develop a new version of the caliber .50 Browning machine gun strictly for aircraft use. The original desired specifications were: a weight of 50 pounds or less, a cyclic rate of 500 to 600 rounds per minute, a muzzle velocity of 2,700 feet per second, capable of penetrating 1-1/8 inch of armor plate at a distance of 25 yards and an ammunition weight of 26 pounds per 100 rounds unlinked. In 1939, the standard .50 caliber M2 aircraft machine gun was capable of firing 600 rounds per minute on a rigid test mount. However, when installed in an aircraft mount, the cyclic rate was reduced by approximately 100 rounds per minute. This was due to the resiliency of the aircraft’s mount, and the heavy ammunition belt loads on the gun’s feed mechanism.A 100 round belt of .50 caliber ammunition weighted 30.25 pounds. The Air Corps later revised their cyclic rate requirement up to 1,000 rounds per minute. To be effective as an aircraft machine gun the big Browning needed to have a faster cyclic rate and an increased belt lift capacity.
Colt, which held John Browning’s patents, undertook a redesign of the gun to meet these requirements only to have each model tested show serious defects. In 1940, engineers at Springfield Armory lightened the barrel of a standard M2 machine gun to 9.5 pounds, and by substituting double driving springs were able to increase the cyclic rate up to 800 rounds per minute. Still, this was far below the desired 1,000 rounds per minute rate specified by the Air Corps. Increasing the cyclic rate of a powerful gun such as the .50 caliber M2 resulted in excessive stress placed upon the barrel and all related parts. Rapid barrel erosion was another problem associated with the high rate of fire. All .50 caliber aircraft barrels had an overall length of 36 inches, which was shorter and lighter than those normally used on the ground guns. This increased the cyclic rate slightly over the ground guns fitted with 45 inch barrels. Chromium plated and eventually chromium barrels with Stellite liners were produced to help in extending barrel life. Many of the mechanical improvements and upgrades implemented into aircraft guns were eventually applied to the ground models.
The rate of fire of aircraft .50 caliber M2 guns produced later during World War II was successfully increased to fire up to a rate as high as 850 rounds per minute. The cyclic rate of the aircraft M2 gun was increased through the use of a lighter bolt, a lighter barrel and a muzzle booster. The M2 caliber .50 aircraft guns used the same basic receiver as the other models of the M2 .50 caliber guns. All of the internal parts that were originally installed in aircraft machine guns were left unfinished. This procedure contributed to a slight increase in the cyclic rate as the surfaces provided less friction. Although the cyclic rate of the M2 Aircraft Basic was deemed satisfactory, research and development continued to increasethe weapon’s firepower and speed.
The most prolific use of the Browning machine gun during World War II was the .50 caliber guns used in aircraft applications. There were as many as fourteen .50 caliber machine guns fitted to U.S. bombers. The guns were used for defending the bombers against enemy fighter aircraft attempting to shoot them down. Tactical formations of bombers were established, positioning each of the aircraft in such a manner that they could defend each other. This was especially important early in the war when U.S. fighter escorts did not have the range to accompany the bombers to the target.
The .50 caliber Browning machine guns slated for aircraft applications were made in both flexible and fixed configurations. Fixed guns were generally installed in the wings or fuselage of fighter and pursuitaircraft and fired remotely by the pilot.Flexible guns were installed in cradles or turrets of bombers, and were considered as part of the aircraft’s structure. Members of the aircrew generally operated the flexible guns. On fighter aircraft, the .50 caliber machine guns were used for offensive air to air engagements and the strafing of ground targets.
Although the cyclic rate was eventually increased to 750 to 850 rounds per minute, the Air Corps still desired a .50 caliber machine gun with a minimum cyclic rate of 1,000 rounds per minute in order to exploit the full potential of the .50 caliber round. Other requirements were: automatic fire control by a trigger motor, right and left-hand ammunition feed, and adaptability for use in either fixed or flexible positions with the least possible trunnion reaction. Many of the Air Corps requirements were eventually met, but the 1,000-plus rounds per minute cyclic rate remained elusive.
This article first appeared in Small Arms Review V10N11 (August 2007) |
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