The Hispano-Suiza HS.404 20 mm autocannon was one of the most widely used aircraft weapons of the 20th century, used by British, American, French, and many other military services.

Development

This cannon was based on the earlier Swiss Oerlikon FF S weapons, which Hispano-Suiza manufactured under license HS.7 and HS.9. In the late 1930s engineer Mark Birkigt designed a new and much improved version with a revised action, much faster rate of fire, and somewhat higher muzzle velocity. The result was the Type 404, or HS.404, which was widely considered the best aircraft cannon of its kind. The 404 was widely used on pre-war French designs.

The new design was adopted by the RAF and FAA in 1941 in a slightly modified form as the Hispano Mk.II (the originals being Mk.I's). It quickly replaced the common Browning .303 machine guns then the standard armament of British places, requiring modifications to the wings of the Supermarine Spitfire and Hawker Hurricane to mount it. By 1943 planes with Brownings were either upgraded or removed from service, and the RAF became an "all-cannon" force.

The gun was also licensed for use in the US as the M1, with both the USAAC and US Navy planning to switch to the 20mm 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 US for comparison, with the differences being primarily that the British version used a slightly shorter chamber that they believed was the cause of the US's problems.

The US declined to modify the chamber their version, but nevertheless made different modifications to create the no-more reliable M2. By late 1942 the USAAC had 40 million rounds of ammunition stored, but the guns were no more reliable. The US Navy had been trying to go all-cannon throughout the war, the conversion never occured. As late as December 1945 the 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 US versions and production levels had ramped up to the point where this was no longer an issue anyway. They upgraded to the Mk V, which was lighter and had a higher rate of fire, although at the expense of some muzzle velocity. The US followed suit with the M3, but reliability problems continued. After World War II the 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 (4½ oz) 20 mm x 110 mm projectile with a muzzle velocity between 840 and 880 m/s (2,755 and 2,888 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 post-war designs. The USAF introduced the M39 revolver cannon to replace the M24, while the Navy instead stuck with the original design with a lighter round for better muzzle velocity in the Colt Mk 12 cannon.