It was widely produced by Oerlikon Contraves and others, with various models employed by both Allied and Axis forces during World War II.

Because the Treaty of Versailles banned further production of such weapons in Germany, the patents and design works were transferred in 1919 to the Swiss firm SEMAG (Seebach Maschinenbau Aktien Gesellschaft) based near Zürich.

The purpose of this development was to improve the performance of the gun as an anti-tank and anti-aircraft weapon, which required a higher muzzle velocity.

In 1930 Oerlikon reconsidered the application of its gun in aircraft and introduced the AF and AL, designed to be used in flexible mounts, i.e. manually aimed by a gunner.

[4] Apart from changes to the design of the guns for wing-mounting and remote control, larger drums were introduced as it would not be possible to exchange magazines in flight.

The 1930s were a period of global re-armament, and a number of foreign firms took licenses for the Oerlikon family of aircraft cannon.

In the air, the Ikaria MG FF was used as armament on a number of German aircraft, of which the most famous is the Messerschmitt Bf 109.

Later in the war, they also equipped fighters including the Zero with the Type 99 Mark 2, a version of the more powerful and faster-firing Oerlikon FFL.

It was not until the Commander-in-Chief of the Home Fleet, Admiral Sir Roger Backhouse, was appointed First Sea Lord that Mountbatten's efforts bore fruit.

Just a few weeks before the fall of France, the Oerlikon factory approved manufacture of their gun in the United Kingdom, under licence.

It became famous in the naval anti-aircraft role, providing an effective defense at short ranges (in practice up to 1.5 km) at which heavier guns had difficulty tracking a target.

The gun was eventually abandoned as a major anti-air weapon due to its lack of stopping power against heavy aircraft and against Japanese kamikaze attacks during the Pacific War.

A handful of corvettes were fitted with the weapon toward the end of the war, but it appeared more commonly on frigates and destroyers at the time.

[6] It is still in use today on some naval units, nominally as a last-recourse anti-air weapon, but mainly used for firing warning shots or incapacitating small vessels.

[citation needed] Unlike most high-powered autocannons, the Oerlikon and its derivatives have a blowback mechanism: the bolt is not locked to the breech of the gun at the moment of firing.

[7] In contrast, 20mm cartridges are far too powerful, and efficient autocannon barrels too long, for this basic system to be practical; so the Oerlikon uses Advanced Primer Ignition (API) to augment the resistance of the bolt.

Synergistically with this, a second advantage of this unusual arrangement is that after firing the bolt and case have a short, but significant, distance to travel rearwards before the bolt-end re-emerges and the case in turn begins to leave the chamber; and this in combination with the retardation of the rearward travel provides sufficient time for gas pressure to drop to the necessary safe level.

[4] This unique chamber and bolt design necessitates the use of a characteristically shaped cartridge: the case has straight sides, very little neck, and a rebated rim.

In a typical single-barrel naval version, it is free-swinging on a fixed pedestal mounting with a flat armored shield affording some protection for the crew.

[9][10] It was also placed experimentally on the battleships Arkansas, Colorado, Maryland, West Virginia, Washington, Massachusetts, and training ship Wyoming.

Although guns with blowback action had played an important part in WW2, it was obvious that something better was needed, especially for the universal demand for a high rate of fire.

As a response to this demand, Oerlikon developed "power reserve loading" action,[12] introducing a gas-operated mechanism to unlock the breech.