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 The MG FF and FF/M saw widespread use in fighters such as the Bf 109 E-3 to F-1, Bf 110 C to F, and Fw 190 A-1 to A-5. Early variants of the Fw 190 (A-1 to A-5) were typically fitted with an inboard pair of MG 151 and an outboard pair of MG FF/M, although the MG FF/M were sometimes removed in the field in order to save weight. The MG FF/M fed from a 60-round drum that required an underwing bulge to fit within the wing (90 rounds in the A-5). From the A-6 onward, the MG FF/M were replaced by a pair of MG 151/20 feeding from 125 round belts, or deleted altogether. The cannon was also fitted to bombers such as the Do 217, Ju 88, He 111, Do 17, as well as many other aircraft, either as aerial defense, or more often for anti-ship and defensive fire suppression. Although the MG FF was often replaced with the 20 mm MG 151/20 from 1941 onwards, it saw a comeback in 1943 as the primary Schräge Musik gun in the Bf 110 (and other) night fighters, as it fit perfectly into the rear cockpit, and muzzle velocity was less