The MK 115 (German: Maschinenkanone 115 "machine cannon 115") was an autocannon developed in Germany in late World War II by Rheinmetall-Borsig for use in aircraft. A single prototype in the late stages of development was captured by the Western allies.
The MK 115 was a gas operated, belt fed weapon, and its breeckblock used a swinging lock operation. It was an unusual development in that although it used a locked breech, it also used a funnel to allow some of the propellant gases to escape out the rear in order to reduce recoil when firing, essentially being an automatic recoilless rifle. The MK 115 was chambered for a 5.5cm round (in common with a few other late-war German designs), but used a partially combustible cartridge, leaving only the base of the cartridge to eject. It had a rifled barrel with an 8°30′ twist.