The basic design was based on the original
Oerlikon 20 mm cannon, in turn based on the
German Empire's
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 the Becker 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 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 at a cyclic rate of 520 rounds per minute, almost double that of the original F and AF models. The gun weighed only . Similar improvements led to the
FF L using a longer 20x101mm round which gave , and the
FF S firing a 20x110mm round at at a slightly slower 470 rounds per minute. 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 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 Werke in Germany started production of the FF F with a slightly more powerful 20x80mm round as the
MG FF, but later introduced a new
mine shell round which was lighter and improved velocity. The resulting MG FF/M was a common weapon until about 1944. Starting in late 1940 these were replaced by the very different Mauser
MG 151/15 and Mauser MG 151/20 autocannon, respectively of 15mm (early) and 20mm calibre (primary production). ==See also==