MG 151 cannon

The MG 151 (MG 151/15) was a German 15 mm aircraft-mounted autocannon produced by Waffenfabrik Mauser during World War II. Its 20mm variant, the 20 mm MG 151/20 cannon, was widely used on German Luftwaffe fighters, night fighters, fighter-bombers, bombers and ground-attack aircraft. Salvaged guns saw post-war use by other nations.
Development and wartime history (MG 151/20).
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 most powerful 20mm Oerlikon of the time, namely the FFS 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 and put in widespread use, but its performance was lackluster.)
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 as the 20 mm MG 151/20 in 1941 to fire a 20 mm cartridge. 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 cartridge to hold the larger explosive shell used in the MG FF cannon, and shortening the length of the cartridge case holding the longer 20 mm shell 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. With an AP projectile the new 20 mm cartridge could penetrate only around 10–12 mm of armor at 300 m and at 60 degrees, compared to 18 mm penetration for its 15 mm predecessor in the same conditions but this was not seen as a significant limitation. The 20 mm version thus 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 explosion rather than armor penetration was taken further with the development of the mine shell, first introduced for the MG FF (in the Bf 109 E-4) and later introduced 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, from the shattering effects of the hexogen explosive in the shells used for both the long-barreled MK 103 and shorter barreled MK 108 cannon. Only 4–5 hits with 20 mm calibre cannon were needed for frontal attacks on four-engined bombers 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 exported to Japan aboard the Italian submarine in August 1943 were used to equip 388 Japanese Kawasaki Ki-61-I Hei 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 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 is photographed on parade in Posen November 1944 with the Wartheland Volksstrum 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 Air Force (AdA) and French Army aviation arm (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 H-21C assault transport helicopters and on Sikorsky HSS-1 Pirate gunship helicopters. French Matra MG 151 20mm 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 151/20 specifications.
Two versions of the 20 mm MG 151 were built. Early guns used a percussion priming system, and later E-models used electrical priming. Some rounds were available with a timer self-destruct and/or tracer (or glowtracer). There were also different types of high-explosive shell fillings with either standard PETN, a mixture called HA41 (RDX and aluminium), and a compressed version where more explosives were compressed into same space using large pressures (XM).
US derivative.
During World War II the US Army produced the .60-caliber T17, a reverse-engineered copy of the German MG151 chambered for an experimental anti-tank rifle round. A speculative order of 5,000 T17 guns was placed but only around 300 of them were built. However none saw service despite the availability of 6 million rounds of .60 caliber ammunition. Almost one million rounds were fired during the T17 testing program. The main US version produced, the T17E3, was made by Frigidaire; it weighed and had a rate of fire of only 600 rounds per minute. Further refinements led to the T39 and T51 versions, but these also did not enter service.
Ammunition.
A cartridge originally based on an armor-piercing round designed in 1939 for use with the experimental T1 and T1E1 anti-tank rifles. It was cancelled in 1944 when it became clear that modern tanks had armor too thick to penetrate with a heavy rifle cartridge. Developments showed that shaped-charged rifle grenades and rocket launchers were the future of infantry anti-tank weapons and the anti-tank rifle concept was abandoned. 
Much like the British attempts to turn their stocks of obsolete .55 Boys anti-tank cartridges into a native-designed heavy machinegun cartridge, the .60-caliber cartridge was repurposed as an auto-cannon cartridge to succeed the older .50 Browning. The ammunition and the T17 cannon were produced from 1942 to 1946 but never proved a substantial improvement over the .50 Browning and the M2HB and M3 heavy machineguns. The cartridge was later shortened and necked-up to produce the 20x102mm Vulcan autocannon round.