20mm L/70 Anti-Air Artillery
    The apparent problems with the 28mm L/74 powered mount, which suffered from its complexity, and from a certain impatience among the naval crowd, which did not (and probably could not) wait for the early problems of the 28mm to be worked out. 
    The Bureau of Ordonance favored a free-swinging gun, which could be used even without any source of power still active on the ship it was on. The only current free-swinging weapon was the .50 caliber machine gun, which, in ships, lacked range and firepower to be more than a retaliating weapon. To replace this weapon, there was need for more firpower and still low enough weight to be handled manually. 
   The most outstanding candidate for this kind of armament was the Swiss Oerlikon company's 20mm gun, which was in service with Great Britian and which had been reviewed for service with the USN before, but rejected on grounds of low rate of fire. 

    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. 

Year of Construction: 1940  
Bore:  20mm  
Weight of gun: 64 - 68 kg / 141 - 150lbs  
Weight of barrel: 21.8 kg / 48 lbs  
Length of gun: 2210mm / 87 ins  
Length of bore: 1400mm / 55.118 ins  
Wt. of projectile: 0.12kg / 0.27lbs  
Max. Range: 4390m / 4800 yds  
Ceiling: 3050m / 10.000 ft at 90°  
Max. Elevation: 90°  
Rate of Fire: 265 rounds/min