The 20 mm Oerlikon was a naval anti-aircraft gun used by Allied navies set up on warship decks during World War II, in single, twin or quadruplet mountings. Designed by the German engineer Reinhold Becker in 1914, it was developed and first manufactured in Switzerland in 1919 by Werkzeug Maschinen Fabrik Oerlikon, but it was soon produced massively, under license, in defferent countries. In the United States of America, the 20mm Oerlikon entered service in 1940 and became part of the armament of US aircraft carriers, battleships, destroyers, and cruisers. It saw combat action in the Pacific theater against Japanese aircraft. It was also used on two-wheeled mobile platforms by ground forces and in both German and Allied fighter and attack aircraft.

The 20 mm Oerlikon was a gas-operated automatic cannon, which had a maximum effective ceiling of 1,100 m (3,600′) and a rate of fire of 480 rounds per minute.  Firing 20x110mm HE, incendiary or tracer ammunition, it could tear out gaping holes in the fuselage of Japanese aircraft or rip off its wing. It had a muzzle velocity of 840 m/s. Traverse: 360º. Elevation: -10º to + 75º. Weight: 66.68 kg.

Row of single-mounted 20mm Oerlikons on the starboard side of a US Navy carrier


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