Hispano cannon

HS.404
HS.404 in the TCM-20 twin anti-aircraft configuration, displayed at the Israeli Air Force Museum.
Type	Aircraft cannon
Place of origin	France
Service history
Wars	World War II Korean War
Production history
Designer	Marc Birkigt
Manufacturer	Hispano-Suiza
Specifications
Weight	43 kg (94 lb 13 oz)
Length	2.52 m (8 ft 3 in)
Barrel length	80 centimeters (31.5 inches)
Cartridge	20×110mm
Calibre	20 mm (0.79 in)
Action	Gas operated, delayed blowback
Rate of fire	700 rounds/min
Muzzle velocity	840–880 m/s (2,800–2,900 ft/s)
Feed system	Drum magazine, belt (later models)

The HS.404 is an originally designed and produced by Hispano-Suiza in the mid-1930s. It was widely used as an aircraft, naval, and land-based weapon by French, British, American, and numerous other military services, particularly during the Second World War. 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 20mm Hispano.

Firing a 20 mm calibre projectile, it delivered a useful 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 multiple 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.

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 able to fire through the propeller shaft 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 an entirely 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 into the barrel about half way along its length hot gas behind the projectile is siphoned off and enters a chamber. Inside the chamber is a piston that presses on a rod running along the top of the barrel back to the bolt, unlocking it and allowing it and the spent cartridge to move backwards. Since the bolt was locked during firing, it could be lighter than that of the Oerlikon, thus facilitating an increase in rate of fire to 700 rounds per minute (rpm), a gain of about 200 rpm. In 1938, Birkigt patented it and started production in their Geneva factory.