sentences:  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.These guns were air-cooled and used a gas blowback recoil system.