The FF were a series of 20mm autocannon 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 French Hispano-Suiza HS.404 (adopted by the British and U.S.), the German MG FF, and the Japanese Type 99 cannon.
The original design, introduced as the FF, fired a 128 gram 20x72mm round with a muzzle velocity of 600 m/s at a cyclic rate of 520 rounds per minute. The gun weighed only 24 kg. The low muzzle velocity was of some concern, so additional developments led to the 30 kg FF L using a longer 20x101mm round which gave 750 m/s, and the 39 kg FF S firing a 20x110mm round at 830 m/s at a slightly slower 470 rounds per minute. The original guns became known as the FF F from this point on.
The MG FF, like all pre-World War II Oerlikon guns, was an API blowback recoil-operated weapon and embodies certain features which are not found in other automatic cannons. The most important of these are: 1) a barrel which does not recoil; 2) a heavy breechblock which is never locked against the breech and actually moves forward when the gun is fired.
The FF F was licensed by the Japanese and produced as the Type 99-1, along with the FF L as the Type 99-2. Hispano-Suiza built the FF S as the HS.7, and slightly improved HS.9. This design was later abandoned by Hispano-Suiza. In the 1930s, Marc Birkigt designed an entirely new gas-operated cannon with a locked bolt, the HS.404, which became one of the best 20 mm weapons of the war. Ikaria in Germany started production of the FF F with a slightly more powerful 20x80mm round as the MG FF, but later introduced a new Minengeschoss round. The resulting MG FF/M was a common weapon until about 1941. Starting in late 1940 these were replaced by the very different Mauser MG 151/20.
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