M39 M39 cannon in the nose of a Brazilian F-5. Type Single-barrel Autocannon Place of origin United States Service history In service 1952–present Production history Designer Ford Motor Company Designed 1951 Manufacturer Pontiac, Ford, and others No. built 35,500+ Variants M39, M39A1, M39A2, M39A3 Specifications Weight 80.9 kg (178.5 lb) Length 1.83 m Caliber 20 mm (0.787 in) × 102 mm Barrels Single barrel (progressive RH parabolic twist, 9 grooves) Action five-chamber revolver Rate of fire 1,500 rpm Muzzle velocity 1,030 m/s (3,300 ft/s)