Rheinmetall BK-5
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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 Boeing B-17 Flying Fortress. Wikipedia
Barrel length: 3.037 m (9.96 ft) l/60
Caliber: 50mm
Feed system: 21 rounds
No. built: approx. 300
Shell: 50×420mm R