Indeed some reason for the adoption of the 20mm may have been the British who had lost their contact with Switzerland in 1940 and who, despite having a production license, did not begin production at home and wanted a U.S. production line, which request was denied because the Oerlikon was not standard US armed forces issue. Before long, and on grounds of seeing it as the best available weapon, BuOrd approved the production on 9th November 1940. 
Reception in the fleet was enthusiastic, the gun being a more than welcome replacement of the .50 caliber, but production could not sufficently quickly replace all of the latter, so that in the early months, most of the air-defenses aboard ships had to rely on the .50-caliber. 

    Indeed the career of the 20mm was quite short. It had extraordinary successes in 1942, 1943 and up to mid-1944, but with the advent of the kamikaze, the 20mm was not adequate anymore. It was, however, retained because of the psychological effect that the ability to fire at the attacker had for the seamen, and twin and quadruple mounts had been tested and installed aboard several ships. 
   The 20mm was fed from a drum atop the gun, as seen in the picture above.