Oerlikon FF
The FF were a series of 20 mm autocannon introduced by Oerlikon in the late 1920s. The name comes from the German term flügelfest, meaning wing mounted, fixed, being one of the first 20 mm guns to be small and light enough to fit into a fighter aircraft's wing. The FF series served as the inspiration for many World War II 20mm cannon, including the French, British and US Hispano HS.404, the German MG FF, and the Japanese Type 99 cannon. The original design, introduced as the FF, fired a 128 gram 20 mm x 72 mm round with a muzzle velocity of 600 m/s and a cyclic rate of 520 rounds per second. The gun weighed only 24 kg, and its belt feed was designed to allow the ammunition to be carried in flat boxes in the wings. The low muzzle velocity was of some concern, so additional developments led to the 30 kg FF L with a 20 mm x 101 mm round with 750 m/s, and the 39 kg FF S firing a 20 mm x 110 mm round at 830 m/s at a slightly slower 470 rpm. The original guns became known as the FF F from this point on. 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 dramatically improved as 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 20 mm x 80 mm round as the MG-FF, but later introduced a new Minengeschoss round made from pressed instead of milled brass that had considerably thinner walls and therefore carried more explosive. The resulting MG FF/M was a common weapon until about 1941. Starting in late 1940 these were replaced by the excellent Mauser MG 151/20, a very different weapon.
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