The MG 151 is a 15mm aircraft cannon that was designed by Waffenfabrik Mauser AG during World War II. The cannon was later redesigned to fire the more powerful 20x82mm round, a slightly longer-necked variant of the round used in the current aircraft autocannon in service with the Luftwaffe, the MG FF.
The development of the MG 151 was started from a request to develop an aircraft cannon that followed closely with current German aerial warfare doctrine, which followed very closely to that of the French, which consisted of having a high-powered autocannon mounted in the cylinder in a V engine firing through the propeller hub. The Oerlikon FFS, the currently most powerful variant of the Oerlikon FF autocannon currently in existence, was incapable of fitting in the cylinders of the engines of the in-service fighters, leading to a contract with Mauser to develop the MG 151.
Production of the 15mm variant began in 1940, and after tests with the Messerschmitt Bf 109F-2 fighter, the cannon was reworked to fire 20x82mm rounds, and from the Bf 109F-4 and onwards the MG 151/20 became the standard offensive armament of the Bf 109 series. However as the preference of explosive ammunition became more evident with the Minengeschoss shell, the MG 151/20 was eventually supplemented as an engine-mounted cannon with the more powerful 30mm MK 103 and MK 108, both with superior fragmentation with it's own Minengeschoss shell capable of downing a bomber in 4-5 hits.
The MG 151's adoption did not entirely remove the MG FF from service, however. It found new life as defensive night fighter armament as "Schräge Musik" (a term given to upward firing autocannons used as defensive armament used in German night fighters).
The only major variant of the MG 151 is the MG 151/20 which fired 20mm shells instead of 15mm rounds.