The Hispano-Suiza HS.404 autocannon was one of the most widely used aircraft weapons of the 20th century, used by British, American, French, and many other military services. Firing a 20 mm diameter projectile, it delivered a useful load of explosive from a relatively light weapon. This made it an ideal aircraft weapon, replacing the multiple 7.62 mm (.30 caliber) machine guns commonly used in military aircraft in the 1930s.
In the meantime, Great Britain had acquired a license to build the HS.404, which entered production as the Hispano Mk.I. Its first used was with the Westland Whirlwind of 1940, providing the Royal Air Force with a powerful cannon-armed interceptor. It was also used in early versions of the Bristol Beaufighter. The Beaufighter highlighted the need for a belt feed mechanism; in the night fighter role the 60-round drums needed to be replaced in the dark by the Wireless Operator, often while the aircraft was maneuvering to keep sight of its quarry. In addition, early trial installations in the Hawker Hurricane and Supermarine Spitfire had shown a tendency for the gun to jam during combat maneuvers, leading to some official doubt as to the suitability of cannons as the sole main armament. This led, briefly, to the Air Ministry specifying 12-gun machine gun armament for its future fighters.
Subsequently a suitable belt-feeding system was developed by the Martin-Baker Aircraft Co. Ltd. and the new design was adopted by the RAF and FAA in 1941 in a slightly modified form as the Hispano Mk.II. Four cannons replaced the eight Browning .303 machine guns in the Hurricane and in tropical versions of the Spitfire, and became standard armament in later fighters. Most other Spitfires had only two cannons, because of technical difficulties (i.e., inadequate gun-heating capacity for the outboard cannon leading to the gun freezing at high altitudes), along with four 0.303 calibre or two 0.50 calibre machine guns.
The gun was also licensed for use in the United States as the M1, with both the United States Army Air Corps (USAAC) and U.S. Navy planning to switch to the 20 mm as soon as sufficient production was ready. A massive building program was set up, along with production of ammunition, in 1941. When delivered, the guns proved to be extremely unreliable and suffered a considerable number of misfires due to the round being "lightly struck" by the firing pin. The British were interested in using this weapon to ease production in England, but after receiving the M1 they were disappointed.
In April 1942 a copy of the British Mk.II was sent to the U.S. for comparison, the British version used a slightly shorter chamber and did not have the same problems as the U.S. version of the cannon. The U.S. declined to modify the chamber of their version, but nevertheless made other modifications to create the no-more-reliable M2. By late 1942 the USAAC had 40 million rounds of ammunition stored, but the guns remained unsuitable. The U.S. Navy had been trying to go all-cannon throughout the war, but the conversion never occurred. As late as December 1945 the Army's Chief of Ordnance was still attempting to complete additional changes to the design to allow it to enter service.
Meanwhile, the British had given up on the U.S. versions and production levels had been ramped up to the point where this was no longer an issue anyway. They upgraded to the Hispano Mk. V, which had a shorter barrel, was lighter and had a higher rate of fire, (desirable in aircraft armament) although at the expense of some muzzle velocity. One of the main British fighters to use the Mk. V was the Hawker Tempest Mk. V Series II, which mounted a total of four. The U.S. followed suit with the M3, but reliability problems continued. After World War II the United States Air Force (USAF) adopted a version of the M3 cannon as the M24, similar in most respects except for the use of electrically primed ammunition.
The Hispano fired a 130 gram (4.58 oz) 20 mm × 110 mm projectile with a muzzle velocity between 840 and 880 m/s (2,750 and 2,900 ft/s), depending on barrel length. Rate of fire was between 600 and 850 rounds per minute. It was 2.36 m (7 ft 9 in) long, weighing between 42 and 50 kg (93 and 110 lb). The British Mk V and American M3/M24 weapons were lighter with higher rates of fire than the early HS.404 guns.
In the post-war era the HS.404 disappeared fairly quickly due to the introduction of revolver cannon based on the German Mauser MG 213. The British introduced the powerful 30 mm ADEN cannon in most of their post-war designs, and the French used the very similar DEFA cannon, both firing the same ammunition. The USAF introduced the 20 mm M39 revolver cannon to replace the M24, while the Navy instead combined the original Hispano design with a lighter round for better muzzle velocity in the Colt Mk 12 cannon.
Hispano Mk. V