MG 151 - Maschinengewehr 151

Mg 151/15 Specifications:
Type: single-barrel automatic cannon
Caliber: 15 mm x 96
Operation: Recoil-operated; short recoil
Length: 1916 mm
Barrel length: 1254 mm
Rifling: 8 grooves, right twist, 1 turn in 16"
Weight (complete): 38.1 kg (84 lb)
Rate of fire: 740 rpm
Effective range: 400 m
Muzzle velocity:
  HE-T, HEI-T: 960 m/s
  AP-T: 850 m/s
  AP WC: 1030 m/s
Projectile types:
  AP-T, weighing 72 g
  AP(WC), weighing 52 g
  HE, weighing 57 g/HE filler: 2.8 g

Mg 151/20 Specifications:
Type: single-barrel automatic cannon
Caliber: 20 mm x 82
Operation: Recoil-operated; short recoil
Length: 1766 mm
Barrel length: 1104 mm
Rifling: 1 turn in 23
Weight (complete): 42.7 kg
Rate of fire: 750 rpm
Effective range: N/A
Muzzle velocity:
  M-Geschoss: 805 m/s
  HE-T, AP: 705 m/s
Round types:
  M-Geschoss
  HE-T, AP

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The MG 151 (MG 151/15) was a 15 mm autocannon produced by Waffenfabrik Mauser starting in 1940. It was in 1941 developed into the 20 mm MG 151/20 cannon which was widely used on many types of German Luftwaffe fighters, fighter bombers, night fighters, ground attack and even bombers as part of or as their main armament during World War II. The 20 mm MG 151/20 was also fitted on the Italian World War II fighter aircraft of the "Serie 5", the most effective Italian fighters of WWII.

The 15 mm MG 151 was found to have lackluster performance as the main gun on Messerschmitt Bf 109 early F-2, and was soon replaced by the 20 mm version to become the standard cannon for the Bf 109F-4 series onwards until it was superseded by the 30 mm MK 108 cannon.

To create the MG 151/20 round, Mauser simply necked out the MG 151/15's case (i.e. enlarged the opening of the case where the shell fits in) to fit a 20 mm shell—which, incidentally, was the same shell used in the MG FF cannon—and shortened the length of the case so that the total length of the complete round was the same for both calibres. These measures simplified conversion of the cannon between calibres, so that it was possible to convert the 15 mm to the 20 mm MG 151/20 simply by changing the barrel and making other small modifications. However, this simple modification-based approach was not without its drawbacks. The relatively short case of the 20 mm round, coupled with the larger and heavier 20 mm projectile cost some muzzle velocity (950 m/s for the 15 mm round vs. 800 m/s for the 20 mm round—a 16% drop). However, in comparison to the earlier MG FF cannon, the MG 151 had a higher muzzle velocity which gave it a more predictable trajectory and higher impact velocity/longer range.

Nevertheless, the extra HE capacity was considered well worth the loss in muzzle velocity. The basic 20 mm HE round, for example, had almost 30% more explosive content by weight than the 15 mm shell. Furthermore, the MG 151/20 also used the Minengeschoß ("mine shell"), which was made using drawn steel (similar to making cartridge cases) instead of being cast, as was typically done to make cannon shells at the time. The result was a shell with very thin yet strong walls, and hence a very large explosive (or incendiary) capacity. Indeed, the 20 mm M-shell carried 6-8 times the amount of explosives contained in the 15 mm shell. 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 statistics data showed that on average 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. While the larger round rapidly replaced its predecessor—the MG 151/15 was phased out in 1942—German engineers continued research into an even heavier cannon that could rapidly demolish heavy enemy bombers.

Two versions of the 20 mm MG 151 were built: one with a percussion priming system and a second E-model with 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.

Eight hundred MG 151/20 were exported to Japan by a submarine in August 1943 and 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, the most powerful Regia Aeronautica fighter aircraft, built around the Daimler-Benz DB 605 engine.

After WWII, 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 and French Army aviation arm (ALAT) utilized MG 151/20 cannon as both fixed and flexible armament in various aircraft, including helicopters. The FAF 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 HSS-1 Pirate gunship helicopters. French Matra MG 151 20mm cannons were used by Portugal, Rhodesia and South Africa fitted to their Alouette III helicopters.

The 15mm caliber is similar to a 14.5mm round, developed in World War 2 for the Soviet PTRD and PTRS antitank rifles and used in post-war heavy machine guns. Recent developments of 14.5mm High Explosive Incendiary rounds may be regarded as a revival of the 15mm cannon concept.