Oerlikon 20 mm cannon
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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 | |
| Mass | 480 kg (without ammunition) |
| Caliber | 20 mm |
| Action | API blowback |
| Rate of fire | 450 rounds per minute |
| Muzzle velocity | 820 m/s |
| Maximum firing 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 of Germany, very early in World War I, and widely produced by Oerlikon Contraves and others. Various models of Oerlikon cannon were used during World War II, and they are still in use today.
History
Original Becker cannon
During World War I, the German Reinhold Becker developed a 20 mm cannon, known now as the 20 mm Becker using the API blowback method of operation. This used a 20 x70 RB 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.
Oerlikon
In 1919, the patent was sold to SEMAG (the Seebach Maschinenbau 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 20 x 70RB cartidge of the Becker gun, but this was later changed to a more powerful 20 x 72RB cartridge. FF stood for Flügelfest meaning "wing-mounted", as it was intended to be mounted on the wings of aircraft. The FF was also manufactured in Japan and used by the Japanese Navy as the Type 99 Model 1.
A modified version of the FF was manufactured by the Ikaria company in Germany. This was called the MG FF and used a 20 x 80RB cartridge. A later version of the MG FF was the MG FFM which was adapted to fire Minengeschoss ammunition, thin-walled round with higher explosive content.
The Oerlikon FFL was mechanically similar to the FF, but used a larger 20 x 100RB cartridge to obtain a higher muzzle velocity. Japan made a modified version with a 20 x 101RB cartridge called the Type 99 Model 2.
The Oerlikon S used a more powerful 20 x 110RB 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.
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 A6M Zero 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 1937-1938 Lord Louis Mountbatten waged a lone campaign within the Royal Navy to set up an unprejudiced trial for the Oerlikon 20 mm 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 1,500 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. All Oerlikon guns imported from Switzerland in 1940 were mounted on various gun carriages to serve as light AA-guns on land.
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 production of the first British-made Oerlikon guns started in Ruislip, London, at the end of 1940; and the first guns were delivered to the Royal Navy 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 and the 3"/70 Mark 26 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.
The Royal Canadian Navy, since renamed to become the Canadian Navy, popularized the use of the Oerlikon gun as an anti-ship and anti-submarine gun - while it was not effective against the armour of most larger ships, it was used extensively and effectively against U-Boats, and on the decks of larger ships. 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.
The Oerlikon was also used as the basis for the Polsten gun, designed by Polish engineers in exile in the United Kingdom. The gun went into service in 1944, and was used well into the 1950s for, among other uses, on Cromwell tanks and early model Centurion tanks.
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).
Description
The Oerlikon cannon and its derivatives feature blowback operation: The bolt is not locked to the breech of the gun on the moment of fire, but moves freely to the rear while the propellant gases propel the projectile forward. To make sure that the projectile has left the muzzle and the gas pressure in the barrel is down to a safe level before the breech opens, the firing pin strikes the primer while the bolt is still travelling forward, so that the gas pressure first has to overcome the forward momentum of the bolt before it can push it to the rear. To give the heavy bolt sufficient forward speed, a large spring is required, which is wrapped around the barrel of the gun. Also, the chamber is longer than needed to contain the case, so that the bolt and case must travel a small distance to the rear before the case extends beyond the face of the chamber. Nevertheless a fairly heavy bolt must be used, which limits the rate of fire.[1]
This design results in the use of a characteristically shaped cartridge: The case has straight sides, very little neck, and a rebated rim. The straight sides allows the case to slide back and forward in the cylindrical chamber. The neck is not supported while this happens and therefore expands when the case is fired, and the rebated rim allows the face of the bolt, with its extractor claw hooked over the rim, to fit within the chamber. To ease the motion of the case, the ammunition needed to be greased, which was a drawback of the Oerlikon cannon. An alternative developed during WWII was the so-called fluted chamber, which had grooves that allowed gun gas to seep between the chamber wall and the case, taking over the role of the grease.[1]
Ammunition feed is typically by a 60-round drum magazine on the top of the gun. During sustained firing, the magazine must be frequently changed, reducing the effective rate of fire. Belt-fed versions of the gun were developed to overcome this limitation. 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 WWII, twin and quadruple Oerlikon mounts were developed, both for army and for navy use.
References
- 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
