<P> The Oerlikon 20 mm anti-aircraft gun was one of the most heavily produced anti-aircraft guns of the Second World War; the US alone manufactured a total of 124,735 of these guns . When activated in 1941 these guns replaced the 0.50 "/ 90 (12.7 mm) M2 Browning MG on a one - for - one basis . The Oerlikon 20 mm AA gun remained the primary anti-aircraft weapon of the United States Navy until the introduction of the 40 mm Bofors AA gun in 1943 . </P> <P> These guns were air - cooled and used a gas blowback recoil system . Unlike other automatic guns employed during World War II the barrel of the 20 mm Oerlikon gun did not recoil, the breechblock never locked against the breech and actually moved forward when the gun fired . This weapon lacked a counter-recoil brake, as the force of the counter-recoil was checked by the explosion of the next round of ammunition . </P> <P> Between December 1941 and September 1944, 32% of all Japanese aircraft downed were credited to this weapon, with the high point being 48.3% for the second half of 1942 . In 1943 the revolutionary Mark 14 Gun Sight was introduced which made these guns even more effective; however, the 20 mm guns were found to be ineffective against the Japanese Kamikaze attacks used during the latter half of World War II . They were subsequently phased out in favor of the heavier 40 mm Bofors AA guns . </P> <P> Arguably the best light anti-aircraft weapon of World War II, the Bofors 40 mm anti-aircraft gun was used on almost every major warship in the US and UK fleet during World War II from about 1943 to 1945 . Although a descendant of German and Swedish designs, the Bofors mounts used by the US Navy during World War II had been heavily "Americanized" to bring the guns up to the standards placed on them by the US Navy . This resulted in a guns system set to English standards (now known as the Standard System) with interchangeable ammunition, which simplified the logistics situation for World War II . When coupled with electric - hydraulic drives for greater speed and the Mark 51 Director (pictured) for improved accuracy, the Bofors 40 mm gun became a fearsome adversary, accounting for roughly half of all Japanese aircraft shot down between 1 October 1944 and 1 February 1945 . </P>

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