MG 151 Cannon - Wartime History (MG 151/20)

Wartime History (MG 151/20)

Production 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. The 20 mm version became the standard cannon for the Bf 109F-4 series onwards until it was superseded by the 30 mm MK 108 cannon.

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 simply by changing the barrel and making other small modifications. A disadvantage of the simplified conversion was reduction of projectile muzzle velocity from 2,800 feet per second (850 m/s) for the 15 mm shell to 2,300 feet per second (700 m/s) for the larger and heavier 20 mm shell. However, the 20 mm MG 151/20 offered more predictable trajectory, longer range and higher impact velocity than the 1,900 feet per second (580 m/s) cartridge of the earlier MG FF cannon.

The increased high explosive capacity of the 20 mm shell was considered more important than the muzzle velocity advantage of the 15 mm shell. Cast steel shells could hold thirty percent more explosive; and the drawn steel MG 151/20 Minengeschoß ("mine shell") had six to eight times the explosive or incendiary) capacity of conventional 15 mm shells. The new 20 mm shell was relatively effective against enemy aircraft, with the possible exception of heavily built bombers such as the B-17 Flying Fortress or Avro Lancaster. German data indicated the 151/20 required an average of 25 hits to down a B-17, while 18-20 hits were required to down other 4-engine bomber types, and only four hits were required to down a single-engine fighter. The MG 151/15 was phased out in 1942 and German engineers continued research into an even heavier cannon for use against heavy enemy bombers.

Eight hundred MG 151/20 exported to Japan aboard the Italian submarine Cappellini in August 1943 were used to equip 388 Japanese 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.