The MK 108 (German: Maschinenkanone—”machine cannon”) was a 30 mm calibre auto-cannon manufactured in Germany during World War II by Rheinmetall-Borsig for use in aircraft.
The weapon was developed as a private venture by the company in 1940 and was submitted to the Reichsluftfahrtministerium (RLM—Reich Aviation Ministry) in response to a 1942 requirement for a heavy aircraft weapon for use against the Allied heavy bombers appearing over German-controlled regions by then. Testing verified that the auto-cannon was well-suited to this role, requiring on average just four hits with its RDX-loaded high-explosive ammunition to bring down a heavy bomber such as a B-17 Flying Fortress or B-24 Liberator and a single hit to down a fighter. In comparison, the otherwise excellent 20 mm MG 151/20 required an average of 25 hits to down a B-17.
The MK 108 was quickly ordered into production and was installed in a variety of Luftwaffe fighter aircraft. It saw first operational service in late autumn 1943 with the Bf 110G-2 bomber destroyers and in the Bf 109G-6/U4.
The MK 108 saw widespread use among fighters tasked with shooting down enemy bombers. Some of the aircraft deploying, or intended to be armed, with the MK 108 were Messerschmitt Bf 109, Messerschmitt Bf 110, Messerschmitt Me 163, Messerschmitt Me 262, Focke-Wulf Fw 190, Focke-Wulf Ta 152, Focke-Wulf Ta 154, Heinkel He 162, Heinkel He 219, Horten Ho 229 and Junkers Ju 388.
The MK 108 was also fitted to night fighters in an unusual installation, called “Schräge Musik” (German (Colloquialism): “Jazz”, literally “awkward music” or “slanted music”). In this configuration, the cannons were mounted in the fuselage, aiming upwards and slightly forwards at an oblique (18 to 30 degree) angle, depending on fitment and aircraft. This allowed the night fighter to attack bombers, almost always undetected, by approaching from underneath the enemy aircraft – many British heavy bombers had neither weapons on the ventral fuselage nor windows for vision. This installation was so effective that discovery and news of its adoption was much slower than usual in reaching British night-bombing forces, as there were rarely any survivors from the attacks to report the new threat. This system was fitted to some versions of the He 219 Uhu, late model Bf110 night fighters, Junkers 88/388 and the Dornier 217N model. It was also fitted more rarely to the (prototype) Focke Wulf Ta-154 & Fw-189 along with the Me262B-2 model. In the latter case this produced a jet fighter with no less than 6 MK108 cannons – with projected FuG218 radar this would have made a very effective night interceptor.