20 mm M2 Hispano Aircraft Autocannon
|Country of Origin||United States|
|Ammunition Weight||0.13 kg|
|Rate of Fire||600 rounds/min|
|Muzzle Velocity||830 m/s|
Contributor: Alan Chanterww2dbaseThe 20mm M2 Hispano aircraft cannon was a development of the French Hispano-Suiza HS.404 built under license in the United States for the USAAC and US Navy. Originally designed by Marc Birkigt, the chief engineer of Hispano-Suiza, in 1933, as a replacement to the Swiss Oerlikon the new weapon employed the locking mechanism which had been patented in 1919 by the American machine-gun designer Carl Swebilius.
In 1938, the French government requested an aircraft based version of the HS.404 to be installed on a wide range of pre-war French fighter plane designs, notably in installations that fired through the drive shaft of the Hispano-Suiza 12Y engine. Drum-fed in earlier versions the cannon's magazine could accommodate only 60 rounds which was something of a drawback. Later a belt-fed system was developed which then became standard on all World War II cannon armed aircraft.
In the build-up to the Second World War the United Kingdom, embarking on a programme to develop cannon-armed fighters, acquired a licence to build the HS.404. Its first use was in the 1940 Westland Whirlwind, and the Bristol Beaufighter nightfighter. A small number of Spitfire fighters were fitted with the 20mm cannon in the latter stages of the Battle of Britain but a tendency for the gun to jam during combat, frequently after only one shot, that the RAF Squadron soon demanded that they be replaced with the eight 0.303-in machine-gun armed version.
By 1941 the British Air Ministry had become concerned that production was becoming inadequate and so a manufacturing license was granted to the United States. The Americans planned a very large building program along with plentiful ammunition production, but the resultant weapons, designated M1, proved to be a disappointment and the British eventually gave up on the American system and set about improving their own model with a shorter barrel that could, with some loss of muzzle velocity, be incorporated into a fighter's slender wing.
Meanwhile, the Americans continued to progress with various redesigns of their own and eventually an improved M2 version, developed by International Harvester, was standardised for use on the Lockheed P-38 Lightning aircraft. The US Navy despite rating the 20mm cannon very highly and facing the need to deal with attacking bombers and Kamikaze aircraft fitted it to few aircraft. There was serious production problems with the American Hispano which gave it a reputation for unreliability. Despite this production would run well into six figures although the American Hispano never managed to achieve an acceptable reliability standard throughout the war.
The advantages of the 20mm Hispano M2, nevertheless, could not be entirely ignored. It was carried by the P-38 along with four Browning 0.50-in machine guns; by the B-29 Superfortress (which had one in the tail position); and by nightfighters such as the P-61 Black Widow which needed a maximum firepower to convert a short firing opportunity into a kill. It was similarly found useful when mounted in the Grumman F6F-5N where the extra firepower was much appreciated for ground strafing.
After the war the US Navy quickly changed over to the 20mm cannon in its improved, faster-firing and more reliable M3 form, but the USAF stayed with the 0.50 Browning guns until after the Korean war. From the mid 1950s the USAF finally began replacing the old Brownings with 20mm cannons, initially with the M39 revolver and then with the M61 rotary - just as most of the rest of the world were switching over to 30mm.
Anthong G Williams, "Cannon or Machine Gun?" (History in the Air - Aeroplane, September 2004) ww2dbase
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