The MG FF was a drum-fed, blowback-operated, 20 mm aircraft autocannon, developed in 1936 by Ikaria Werke Berlin of Germany. It was a derivative of the Swiss Oerlikon FF F cannon (its FF suffix indicating Flügel Fest, for a fixed-mount, wing location from the Swiss original), with the Oerlikon FF design itself a development of the Imperial German World War I Becker 20 mm cannon, and was designed to be used in space-limited, fixed mountings such as inside aircraft wings, although it saw use as both an offensive and a defensive weapon, in both fixed and flexible format. It saw widespread use in those roles by the German Luftwaffe, particularly during the early stages of World War II, although from 1941 onwards it was gradually replaced by the Mauser firm's 20 mm MG 151/20, which was lighter, and had both a higher rate of fire and muzzle velocity. Development MG FF stands for Maschinengewehr Flügel Fest, which translates into "machine gun, wing, fixed"; this reflects the fact that in Luftwaffe practice guns of 20 mm or less were designated as "machine guns" (maschinengewehr) as opposed to larger "machine cannons", or autocannons, which were "MK", for maschinenkanone. The "wing, fixed" part reflects the fact that the primary motivation behind its design was to create a 20 mm caliber weapon that was compact and light enough to be mounted in the wings of aircraft, especially fighters. Compared to rival designs, such as the Hispano-Suiza HS.404 – which had been developed from the larger Oerlikon FF S – the MG FF had some disadvantages, such as low rate of fire and low muzzle velocity, as well as limited ammunition storage in its drums. On the other hand, it was much lighter and shorter. Even with its compact size, wing installation on the Messerschmitt Bf 109 and Focke-Wulf Fw 190 fighters was not easy, as the drum required substantial space, and as a consequence the ammunition storage was initially reduced to 60 shells per drum. An ammunition drum of 90-round nominal capacity was developed for the Fw 190 A-5, and retrofitted to some earlier variants. There were also experiments with belt feedings. The MG FF was adapted to fire a new type of high-capacity, high-explosive "mine shell" that featured a projectile with thinner walls that allowed increased explosive charge. This projectile was lighter and thus had a higher muzzle velocity than the previous ammunition;