The 20mm Oerlikon is a Swiss designed auto cannon used for high angle close range anti-aircraft fire, with an effective rang of 1,00 to 1,200 yards (915 - 1100m).
The breech is not locked at the time of discharge and the round is fired a fraction of an inch before it is fully home in the chamber, the neck of the case swelling to form a gas seal.
Muzzle velocity 2,725 ft/sec (830m/s) with a rate of fire 465 to 480 rounds per minute. The 20mm rounds are loaded into magazines holding 60 rounds.
The last 2 rounds loaded are usually practice rounds so that if there is insufficient time to remove the muzzle cap the non-explosive rounds can blast through the cap.
The Mark II gun was made in Britain and in the USA principally for naval use.
This animation shows the British Mark II. Models made in the USA are marked Mark II U.S.N or Mark IV U.S.N.
There is no difference between these Marks and all three are interchangeable.
