oerlikon 20mm gun
The Oerlikon 20mm autocannon was one of the most common anti-aircraft guns of World War II, used by both Allied and Axis forces on land, installed into aircraft, and mounted onto naval vessels. The latter is where the Oerlikon was most commonly seen, as Allied navies adopted the Oerlikon wholesale to replace smaller-caliber antiaircraft weapons such as the .50 caliber Browning M2 in 1942. However, the rapid pace of wartime aircraft development outpaced the Oerlikon's ability to effectively destroy enemy aircraft, and the gun was withdrawn from service by the end of the 1940s.
It is most often seen in a single mount, though double and even quadruple mounts were developed and used. The Oerlikon can be easily identified visually by the large piston shroud around the barrel that facilitated its blowback operation. Shipboard mounts tended to have a large drum magazine out the top and a curved brace to fit into the operator's shoulders while firing, as well as a gun shield to protect the loaders.