The pre-war German doctrine for arming single-engine
fighter aircraft mirrored that of the French. This doctrine favoured a powerful autocannon mounted between the cylinder banks of a
V engine and firing through the propeller hub, known as a
moteur-canon in French (from its first use with the
Hispano-Suiza HS.8C engine in World War I, on the
SPAD S.XII) and by the cognate
Motorkanone in German by the 1930s. The weapon preferred by the French in this role was the
20 mm Oerlikon FF S model, but this proved too big for German engines. Mauser was given the task of developing a gun that would fit, with a minimum sacrifice in performance. As a stop-gap measure, the
MG FF cannon was developed from the Oerlikon FF and put in widespread use, but its performance was lackluster. A-5 interceptor two MG 151/20 were fitted in wing roots Production of the MG 151 in its original 15 mm calibre format began in 1940. After combat evaluation of the 15 mm cartridge as the main armament of early
Messerschmitt Bf 109F-2 fighters, the cannon was redesigned with a larger cartridge as the 20 mm MG 151/20 in 1941. Combat experience showed that a more powerful explosive shell was preferable to a higher projectile velocity. The MG 151/20 cartridge was created by expanding the neck of the MG 151/15 cartridge to hold the larger explosive
shell used in the MG FF cannon, and shortening the length of the cartridge case to match the overall length of the original 15 mm cartridge. These measures simplified conversion of the 15 mm to the 20 mm MG 151/20, requiring only a change of barrel and other small modifications. A disadvantage of the simplified conversion was reduction of projectile
muzzle velocity from for the 15 mm shell to for the larger and heavier 20 mm shell. A 20 mm AP projectile could penetrate only 10–12 mm of armor at 300 m (at 60 degrees), compared to 18 mm penetration for the 15 mm under the same conditions but this was not seen as a significant limitation. The 20 mm version became the standard inboard cannon from the Bf 109F-4 series. The 20 mm MG 151/20 offered more predictable trajectory, longer range and higher impact velocity than the cartridge of the earlier MG FF cannon. The MG FF was retained for flexible, wing and upward firing
Schräge Musik mounts to the end of the war. The German preference for explosive power rather than armor penetration was taken further with the development of the mine shell which had been first introduced for the MG FF (in the Bf 109 E-4) and later for the MG 151/20. Even this improvement in explosive power turned out to be unsatisfactory against the four-engine bombers that German fighters were up against in the second part of the war. By German calculations, it took about 15–20 hits with the MG 151/20 to down a heavy bomber but this was reduced to just 3–4 hits for a 30 mm shell with the
shattering effects of its
hexogen explosive filling used by the long-barreled
MK 103 and shorter barreled
MK 108 cannon. Only four or five hits with 20 mm calibre cannon were needed for frontal attacks on heavy bombers (
Boeing B-17 Flying Fortress and
Consolidated B-24 Liberator) but such attacks were difficult to execute. The 30 mm MK 108 cannon thus replaced the MG 151/20 as the standard, engine-mount
Motorkanone centre-line armament starting with the Bf 109 K-4 and was also retrofitted to some of the G-series. Eight hundred MG 151/20 were exported to Japan aboard the Italian submarine in August 1943 and used to equip 388 Japanese
Kawasaki Ki-61-I
Hien fighters. The 20 mm MG 151/20 was also fitted on the
Macchi C.205, the
Fiat G.55 and
Reggiane Re.2005 of the Italian
Regia Aeronautica and
IAR 81B and 81C of the Romanian Royal Air Force. An unknown number of cannons were converted for usage in the ground use role in early 1945, predominantly within
Volkssturm units serving in the
Posen area. Its effectiveness in this role are unknown but it was photographed on parade in Posen November 1944 with the
Wartheland Volkssturm units.
Postwar use After World War II, numbers of ex-Luftwaffe MG 151/20 cannon were removed from inventory and from scrapped aircraft and used by various nations in their own aircraft. The
French ''Armée de l'air (AdA) and
French Army aviation arm (Aviation légère de l'armée de Terre, ''ALAT) used MG 151/20 cannon as fixed and flexible armament in various aircraft, including helicopters. The AdA and ALAT jointly developed a rubber-insulated flexible mount for the MG 151/20 for use as a door gun, which was later used in combat in Algeria aboard several FAF/ALAT
Piasecki H-21C assault transport helicopters and on
Sikorsky H-34 gunship helicopters. French Matra MG 151 20 mm cannons were used by Portugal and
Rhodesia fitted to their Alouette III helicopters, while
Denel designed its
own variant for the
South African Air Force. == MG 151 applications ==