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Oerlikon 20 mm cannon
An Oerlikon cannon on HMAS Castlemaine
Type	Autocannon
Place of origin	Switzerland
Service history
Wars	World War II
Production history
Designer	Reinhold Becker
Designed	World War I
Manufacturer	Oerlikon
Variants	MG FF
Specifications
Weight	480 kg (without ammunition)
Caliber	20 mm
Action	API blowback
Rate of fire	450 rounds per minute
Muzzle velocity	820 m/s
Maximum range	2,000 meters against aerial targets

The term "Oerlikon 20 mm cannon" refers to a series of autocannons, based on an original designed by Reinhold Becker during World War I. Various models of Oerlikon cannon were used during the Second World War, and they are still in use today.

Contents 1 History 1.1 Original Becker cannon 1.2 Oerlikon 1.3 World War II 1.4 Polsten gun 2 Description 3 References 4 See also

During World War I, Reinhold Becker developed a 20 mm cannon using the API blowback method of operation. This used a 20x70RB cartridge and had a cyclic rate of fire of 300 rpm. It was used on a limited scale as an aircraft gun and an anti-aircraft gun towards the end of that war.

In 1919, the patent was sold to SEMAG (the Seebach Machinenbau Aktien Gesellschaft) in Switzerland, who produced more powerful models. SEMAG was bought by Werkzeug Maschinenfabrik Oerlikon, another Swiss company, shortly afterwards.

The smallest Oerlikon gun was the Oerlikon FF which originally used the 20x70RB cartidge of the Becker gun, but this was later changed to a more powerful 20x72RB cartridge. FF stood for Fluegelfest meaning "wing-mounted", as it was intended to be mounted on the wings of aircraft.

The Oerlikon FF was manufactured in Japan and used by the Japanese Navy was the Type 99-1.

A modified version of the FF was manufactured by the Ikaria company in Germany. This was called the MG FF and used a 20x80RB cartridge. A later version of the MG-FF was the MG FFM which was adapted to fire Minengeschoss ammunition.

The Oerlikon FFL was mechanically similar to the FF, but used a larger 20x100RB cartridge to obtain a higher muzzle velocity. Japan made a modified version with a 20x101RB cartridge called the Type 99-2.

The Oerlikon S used a more powerful 20x110RB cartridge and was intended as an aircraft gun. It weighed 62 kg and could fire at 280 rpm. This was seen as being too heavy and too slow-firing, so Oerlikon developed it over the years to produce the Oerlikon FFS which weighed only 39 kg and could fire at 470 rpm.

The Oerlikon SS was also based on the Oerlikon S, and it is this gun which was famously used as a naval anti-aircraft weapon during World War II.

The Germans and the Japanese used their derivatives of the Oerlikon cannons extensively. Among others, they were used on such famous aircraft as the Mitsubishi A6M Zero Fighter and the Messerschmitt Bf 109 as well as on ships and as ground-based equipment.

Initially the Oerlikon was not looked upon favorably by the Royal Navy as a short-range anti-aircraft gun. All through 1937 and 1938 Lord Louis Mountbatten waged a lone campaign within the Royal Navy to set up an unprejudiced trial for the Oerlikon 20mm gun, but it was all in vain. 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. During the first half of 1939 a contract for 1500 guns was placed in Switzerland. However, due to delays and then later the Fall of France in June 1940 only 109 guns reached the United Kingdom.

Just a few weeks before the Fall of France, the Oerlikon factory approved manufacture of their gun in the United Kingdom, under license. The Royal Navy managed to smuggle out the necessary drawings and documents from Zürich. The first British-made Oerlikon guns were produced in Ruislip, London, at the end of 1940; delivery to the Royal Navy began in March or April, 1941.

The Oerlikon gun was fielded in United States Navy ships starting in 1942, replacing the M2 Browning machine gun, which lacked range and firepower. It became famous in the naval anti-aircraft role, notably against Japanese kamikaze attacks during the Pacific War. The gun was eventually abandoned as a major anti-air weapon due to its lack of stopping power against heavy aircraft, largely superseded by the Bofors 40 mm gun. It did, however, provide a useful increase in firepower over the .50 cal machine gun when adapted and fitted to some aircraft; however, it had some problems with jamming in the ammunition feed.

It is still in use today on some naval units, theoretically as a last-recourse anti-air weapon, but mainly used for police shots (warning shots or incapacitating shots).

The Polsten gun was based on the Oerlikon.

The Oerlikon 20 mm cannon is a single-barrelled cannon with a large spring coil surrounding the barrel. Ammunition feed is by a 60-round drum magazine on the top of the gun. A drawback of this weapon was that it requires greased ammunition. A trigger in the right-hand grip controls fire. Used cartridges are ejected from below the breech.

Different nations and services operated a number of mounting types for the same basic gun. 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. The cannon is aimed and fired by a gunner using, in its simplest form, a ring-and-bead sight. The gunner is attached to the weapon by a waist-belt and shoulder supports. For this reason, some mountings existed with a height-adjustment feature to compensate for different sized gunners . A "piece chief" designates targets and the feeder changes exhausted magazines. During sustained firing, the magazine must be frequently changed, reducing the effective rate of fire.

• Campbell, John. Naval Weapons of World War Two. Annapolis: Naval Institute Press, 1985.
• Heller, Daniel. Zwischen Unternehmertum, Politik und Überleben. Emil G. Bührle und die Werkzeugmaschinenfabrik Oerlikon, Bührle & Co 1924–1945. Verlag Huber: Frauenfeld 2002.
• Williams, Anthony G. Rapid Fire: The development of automatic cannon, heavy machine guns and their ammunition for armies, navies and air forces. Shrewsbury: Airlife Publishing LTD, 2003. ISBN 1-84037-435-7
• Pawle, Gerald. Secret Weapons of World War II. New York: Ballantine Books, 1978. ISBN 0-345-27895-X

• 20 mm modèle F2 gun
• M242 Bushmaster
• Polsten