The HS.404 is an autocannon originally designed and produced by Spanish/French company Hispano-Suiza in the mid-1930s. It was widely used as an aircraft, naval and land-based weapon by French, British, American and other military services, particularly during World War II. The cannon is also referred to as Birkigt type 404, after its designer Marc Birkigt and later versions based on British development are known as 20 mm Hispano. Firing a 20 mm calibre projectile, it delivered a significant load of explosive from a relatively light weapon. This made it an ideal anti-aircraft weapon for mounting on light vehicles, as well as a fighter aircraft gun, replacing the 7.62 mm (.30 calibre) and .303 inch (7.7 mm) machine guns commonly used in military aircraft of the 1930s. The HS.404 was produced by the French subsidiary of Hispano-Suiza, and under license by a variety of companies in other countries. Development From Oerlikon to Hispano The first widely used 20 mm aircraft cannon was the Becker model, introduced into German service in World War I. The Becker introduced the advanced primer ignition blowback (API) design for autocannons, a concept that was quickly taken up by other companies. Notable among these was the Swiss Oerlikon FF S, which was based on the Becker but introduced a number of improvements. In the 1930s, Hispano-Suiza was asked to develop a 20 mm cannon to fire through the propeller shaft (as a moteur-canon) of a gear-reduction inline aviation engine like the Hispano-Suiza 8BeC. They took out a license on the Oerlikon FF S and made minor modifications to produce the Hispano-Suiza Automatic Cannon Type HS.7 and HS.9. Shortly after production began, the Hispano-Suiza and Oerlikon companies disagreed over patent rights and their business connection came to an end. In 1933, the chief engineer of Hispano-Suiza, Marc Birkigt, began work on the design of a new weapon to replace the Oerlikon contract, based on a locking mechanism patented in 1919 by the American machine-gun designer Carl Swebilius. The result was the Type 404 or HS.404. While the HS.404 resembled the parent Oerlikon FF S in many respects, its repeating mechanism was a gas-operated locking bolt. On firing the 404, when the projectile passes a hole cut in the barrel, about half way along its length, high pressure gas behind the projectile is siphoned off and operates a piston that drives a rod, running along the top of the