Oerlikon 20mm

Oerlikon 20mm cannons are a series of autocannons based on a design by Reinhold Becker from WWI. Various models were used during WWII on both sides, and they are still used today. 
The Royal Navy adopted Oerlikons relatively late compared to the Germans and the Japanese, not signing a contract with the Swiss firm until 1939. The contract was for 1,500 units, of which only 109 were delivered due to delays, and then the Fall of France in June 1940. Oerlikons imported from Switzerland in 1940 were mounted on various gun carriages to serve as light anti-aircraft guns on land.
Oerlikon approached the British government for permission to relocate production to the United Kingdom, which was granted immediately. The Royal Navy assisted the company in sneaking technical drawings and documents out of Zurich, and by the end of 1940, British-made 20mm Oerlikon autocannons were coming off the line in the new Ruislip factory. The first delivery of these was made by the Royal Navy in the spring of 1941.
The RCN popularized the use of Oerlikons against U-boats and against the decks of larger ships that these guns otherwise weren’t very effective against. A handful of corvettes were fitted with them near the end of the war, but they were used more on frigates and destroyers.
WWII Oerlikons were single-barrelled weapons operated by blowback, with ammunition fed through the top and empty cartridges ejected from below. The triggers were on the right handle and the aiming mechanisms were often simple ring-and-bead sights.
Starting in 1942, 20mm Oerlikons were being used by the US Navy to replace the M2 Browning machine gun as the standard for short-range anti-aircraft weapons. Though they lacked stopping power against heavy aircraft, they played a critical role in the Pacific War, especially against kamikaze aircraft that would get close to American ships. It also provided a useful increase in firepower over the .50 calibre machine gun when fitted to aircraft, though it had problems with the ammunition feed jamming.

Reference Links:
Haze Gray – Canadian Navy Gun Systems
Canadian War Museum – Anti-Aircraft Gun and Crew in Action