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; this also entailed that it generated less recoil than earlier projectiles requiring a modification of the recoil mechanism. With this modification it could fire the new mine shell, but accidentally using the heavier MG FF ammo could damage the gun. In the interest of avoiding such errors, the weapon was redesignated the
MG FF/M. It was introduced with the Bf 109 E-4 and Bf 110 C-4 in summer 1940. The MG FF fired a 134 g projectile with a muzzle velocity of some 600 m/s and a rate of fire of about 530 rounds per minute. The MG FF/M fired a 90 g HE/M (high explosive mine shell) projectile with a muzzle velocity of c. 700 m/s and a rate of fire of c. 540 rounds per minute. AP, HE and incendiary projectiles were also available (115 to 117 g projectiles, 585 m/s, c. 520 rpm) because the mine-shot was not capable of holding incendiary or tracer parts. There were also different types of high-explosive mine shell fillings with either standard
Pentrit A (
PETN +
Aluminium), a mixture called
HA 41 (
RDX +
Aluminium powder)(the latter had a 40 percent increased high explosive and incendiary effect). == Operational use ==