According to the NavWeaps.com website, the 20mm Oerlikon was Widely used by many nations, this 20 mm automatic weapon originally designed by the Swiss firm of Oerlikon was probably produced in higher numbers than any other AA weapon of World War II.

In 1937 the British Admiralty initiated tests to find a weapon suitable for arming merchant ships and minor warships against close range air attacks. They rejected the Oerlikon Model 1934, but in 1938 the Admiralty informed Oerlikon that if they could raise the muzzle velocity and demonstrate that the weapon could be used and maintained by non-specialist personnel, such as fishermen and merchant seamen, then it would be acceptable. Oerlikon made the necessary changes and the first prototypes of the new design were delivered late in 1939. These were immediately accepted into service as the 20 mm Mark I and Britain placed large orders with Oerlikon and obtained a manufacturing license. However, only a few additional guns were delivered prior to the German occupation of France, which cut off the supply route. This is basically why so few British ships had Oerlikon guns during the early part of the war, with the official USN BuOrd history stating that the Royal Navy had only 100 Oerlikons at sea in November 1940.

Shortly before France fell, the British took advantage of their manufacturing license with Oerlikon to obtain a set of production drawings. These were brought back from Switzerland by Stewart Mitchell, who had previously been Inspector of Naval Ordnance Contracts at the Oerlikon factory in Zurich. Mitchell, together with the famous ordnance expert Charles Goodeve and with Cmdr. S.W. Roskill (then working in the Admiralty Staff Division and later the famous Capt. Roskill, author of "The War at Sea") set up a factory at Ruislip to produce Oerlikon guns. "Considerable difficulties" with equipment and labor had to be overcome before deliveries of the British version of this gun, designated as the 20 mm Mark II, began in the fall of 1941. In November 1941, the battleship HMS Duke of York was commissioned with six of these weapons, which I believe to have been the first warship to carry British-produced Oerlikon guns "as completed."

Between December 1941 and September 1944, 32% of all Japanese aircraft downed by the USN were credited to this weapon, with the high point being 48.3% for the second half of 1942. In 1943 the revolutionary Mark 14 Gunsight was introduced which made these guns even more effective. This gunsight was developed by Dr. Charles Draper of MIT, who calculated that since the guns fired at relatively short ranges, a crude but simple and effective relative-bearing system could be used to control these weapons. The Mark 14 gunsight used two gyros to measure vertical and lateral rate of change and with these calculated the lead angle to the target aircraft and then projected an off-set aiming point for the gunner. Use of the Mark 14 did require that an electric power connection be provided to the formerly free-standing mountings. This gunsight was later adopted as part of the Mark 51 director which was used to control the 40 mm Bofors, greatly increasing their effectiveness. Postwar, the Mark 14 was replaced by the Mark 20 Gun Sight, which was a lighter, simpler design. The Mark 20 was ready to use in ten seconds after being switched on while the Mark 14 took three minutes.

As mentioned, there are 18 mounts included included.  They come as one part, no assembly is required.  That will make them easier to install, but perhaps more difficult to paint. 

These weapons are really small, with very thin shields and gun barrels.  Also included are shoulder rests and gun sights.  The most difficult part of using these may be removing them from the print raft without any damage. 

This is Black Cat Models set AC700001c, 20mm Oerlikon Mk.4 Gun with Mk.14 Gunsight (x18).  It retails for about $5.30 USD, and is available directly from Black Cat Models - who I'd like to thank for this review sample - or from some of our other fine sponsors.