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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 15 mm weapon was found to have lackluster performance as the main gun on Messerschmitt Bf 109 early F-2 and F-3 series aircraft^{[citation needed]}, and was soon replaced by the 20 mm version to become the standard cannon for the Bf 109F-4 series onwards until it was superseded by the 30 mm MK 108 cannon^{[citation needed]}.

To create the MG 151/20 round, Mauser simply necked out the MG151/15'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).^{[citation needed]}

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 MG151/20 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 round proved to be highly effective, with German statistics data showing that on average 18-20 hits were required to down a 4-engine bomber while four hits were required to down a single-engine fighter. As such, the larger round rapidly and completely replaced its predecessor - the MG151/15 was phased out in 1942.

Compared to the earlier Oerlikon MG FF cannon, the MG 151 had a higher muzzle velocity which gave it a more predictable trajectory and higher impact velocity/longer range.

• Rifling: 8 grooves, right hand twist, 1 turn in 16
• Round types:
  • AP weighing 66 g.
  • HE weighing 51 g. HE filler: 2.8 g

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 (XM).

• Rifling: 1 turn in 23
• Round types:
  • AP - round weight of 117 g.
  • HE - round weight of 115 g. HE filler: 3.6 g
  • HE(M) - Minengeschoß ("mine shell") - round weight of 92 g. HE filler: 18 g
  • HE(XM) - round weight of 104g. HE filler: 25 g
  • Incendiary, with either phosphorus and thermite filling.
  • API (Armor piercing incendiary).

• WW2 Luftwaffe aircraft weapons

• WWII era US Military report on the weapons