The Rheinmetall Bordkanone 5, or BK-5, was a WWII-era German 50 mm autocannon primarily intended for use against Allied heavy bombers, such as the United States Army Air Forces's (USAAF) Boeing B-17 Flying Fortress.

The shells had a high muzzle velocity and significant kinetic energy, allowing them to be fired from distances outside the range of the bombers' defensive guns.

They were installed as Umrüst-Bausätze ("factory modification") 4 in the Me 410 A-1/U4, and, experimentally, in two Messerschmitt Me 262 A-1a/U4 jet fighter prototypes (though these were not used operationally),[3] as the MK 214A cannon of similar caliber was not yet available.

An experimental fitment of the BK 5 in an undernose Bola (or "dustbin") mount on a small number of Heinkel He 177A-3 heavy bombers was part of a small force of the bombers that was given the task of suppressing Flak on the Eastern Front near Stalingrad early in the winter of 1942–1943 as the A-3/Rüstsatz 5 version, allegedly nicknamed the Stalingradtyp.

[3] According to the account of the engagements against the USAAF by II./ZG 26 from late February through mid-April 1944, the 53 Me 410 Hornisse of that Zerstörergruppe equipped with the BK 5 claimed a total of 129 B-17 Flying Fortress and four Consolidated B-24 Liberator heavy bombers, destroyed over five or six interceptions while losing nine of their own Me 410s.