BK 5 cannon

The Bordkanone 5, or BK 5 for short, was a 50 mm autocannon intended primarily for use against Allied heavy bombers, especially the United States Army Air Forces's (USAAF) combat box heavy bomber flight formations, so that defending Luftwaffe fighter aircraft could fire from a great enough distance to make the USAAF bomber's heavy defensive firepower ineffective against defending German fighter aircraft. Rheinmetall was given a contract in 1943 to adapt the 50 mm KwK 39 tank gun, from the Panzer III tank, for aerial use in the twin-engined Me 410 Hornisse bomber destroyer. They were installed as Umrüst-Bausätze (Factory Modification) 4 in the Me 410 A-1/U4, and experimentally, in two Me 262 A-1a/U4 jet fighter prototypes (though these were not used operationally), as the MK 214 cannon of similar caliber was not yet available. The semi-circular magazine held 21 rounds. Approximately 300 were produced and it saw only limited action, most notably in the Me 410 A-1/U4 aircraft that served with the II. Gruppe of Zerstörergeschwader 26 (ZG 26). It was also mounted on the Junkers Ju 88 P-4 night attack aircraft. Intended for long-range shots, the cannon was given a telescopic sight in addition to the Me 410's standard Revi C12C gunsight. This proved to be more of a hindrance than a help in the turning fights in which the Me 410s often found themselves, as the maneuvering targets easily escaped from the telescopic sight's small field of view.

As installed in the Me 262, the cannon was found to be prone to jamming, and if fired at night the BK 5's muzzle flash tended to temporarily blind the pilot's night vision.

According to the account of the engagements against the USAAF by II./ZG 26 from late February through mid-April 1944 mentioned at a German language website, the 53 Me 410 Hornissen of that Zerstörergruppe equipped with the BK 5 - as the Umrüst-Bausätze factory modification designated /U4 for the Me 410 series of aircraft - were said to have to shot down a total of 129 B-17 Flying Fortress and four B-24 Liberator heavy bomber aircraft, distributed over a series of five or six interceptions, all while losing only nine of their own Me 410s.