Oerlikon FF

The FF were a series of 20mm introduced by Oerlikon in the late 1920s. The name comes from the German term Flügel Fest, meaning wing mounted, fixed, being one of the first 20mm guns to be small and light enough to fit into a fighter aircraft's wing. The FF series inspired many 20mm cannon used in World War II, including the Hispano-Suiza HS.404 (adopted by the French, British and U.S. forces), the German MG FF, and the Japanese IJNAS's Type 99 cannon.

The basic design was based on the original Oerlikon 20 mm cannon, in turn based on the Becker 20 mm cannon of World War I. In the post-war period, Oerlikon had developed the original design to produce three autocannons with increasingly powerful rounds; the original model, later known as the "F", used a 20x70mm round, the newer "L" model used a 20x100 round, and finally the "S" model used a 20x110. All of these weapons used the API blowback recoil-operated operational principle.

In 1935 Oerlikon started development of versions suitable for use as wing-mounted guns. In addition to various changes needed for remote firing and cartridge ejection, the new series focused primarily on rate-of-fire. The original "FF" used a slightly upgraded 128 gram 20x72mm round with a muzzle velocity of 600 metres per second (2,000 ft/s) at a cyclic rate of 520 rounds per minute, almost double that of the original F and AF models. The gun weighed only 24 kilograms (53 lb). Similar improvements led to the 30 kilograms (66 lb) FF L using a longer 20x101mm round which gave 750 metres per second (2,500 ft/s), and the 39 kilograms (86 lb) FF S firing a 20x110mm round at 830 metres per second (2,700 ft/s) at a slightly slower 470 rounds per minute. The original guns became known as the FF F from this point on.

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