20mm Oerlikon Light Antiaircraft Gun

Photograph of 20mm Oerlikon AA gun

National Archives #80-G-71586


Ammunition type Contact fused HE fixed shell, with or without tracer.
HE/incendiary and SAP rounds were also available.
The complete round (projectile plus cartridge) weighed about 0.531 lb (0.241 kg) and was up to 7.18 in (18.2 cm) long.
Projectile weight 0.272 lb
0.123 kg
Velocity 2740 feet/s
835 m/s
Maximum elevation       90 degrees
Range 4800 yards
4390 meters
Altitude 10,000 feet
3050 meters
Rate of fire 480 rounds per minute

The Swiss Oerlikon is usually regarded as a successful design, and it was certainly a vast improvement over heavy machine guns firing solid shot. It won high praise for its performance at the Battle of the Eastern Solomons, where it accounted for at least half of all the Val dive bombers that attacked the American carriers. It was calculated that by September 1944 the Oerlikon had accounted for 32 percent of all identifiable antiaircraft kills by the Pacific Fleet.

The action of all Oerlikon variants was blowback with advanced primer ignition. A blowback action is one in which just the bolt, rather than the entire barrel mechanism, is driven back by the recoil from a fired round. The empty cartridge case is ejected when the bolt reaches maximum recoil, then a powerful spring returns the bolt to its forward position while loading a fresh round into the chamber. Advanced primer ignition means that the primer of the fresh round is ignited while the bolt is still moving forward. This balances the recoil against the momentum of the moving bolt in a way that permits a higher chamber pressure than with simple blowback while better smoothing the recoil forces. This allows a lighter, simpler gun to fire a more powerful round.

The U.S. Navy purchased two early models of the weapon in 1935, but was unimpressed by the muzzle velocity and rate of fire, preferring to continue development of the 0.50 Browning as a naval antiaircraft weapon. However, the Japanese adopted the Oerlikon as an aircraft weapon, and the license fees ironically kept Oerlikon from bankruptcy and permitted further development of the weapon. The British adopted the Oerlikon in 1937 due in part to its ease of use by minimally trained gunners, such as merchant seamen. With the collapse of France in June 1940, the British began to seek contracts for production of the Oerlikon in the United States. There was reluctance by the U.S. government to approve manufacture of a weapon not being used by American forces, but intense British pressure combined with a favorable report on the weapon by W.H.P. Blandy led to its adoption by the U.S. Navy on 8 November 1940. However, production did not begin until late 1941, and only 379 weapons had been produced by the time war broke out in the Pacific. Manufacture ramped up rapidly thereafter, and by April 1942 there were sufficient weapons to rearm the entire U.S. Fleet, meet new construction requirements, and begin supplying the weapon to auxiliaries and merchant ships. Further production increases were ordered in early 1943 to equip landing craft. Production peaked at 4,693 mounts in September 1943, and total production was 124,734 mounts. Initially costing about $7000 per gun, improvements in mass production brought the price down to $1658 by the end of the war.

The Oerlikon was chosen by the Navy over the competing Hispano 20mm preferred by the Army for a number of reasons. Although the Oerlikon had a significantly slower rate of fire than the Hispano, it was a much more robust weapon, requiring less skilled maintenance. Its barrel could be changed in just 30 seconds, an important consideration for a weapon likely to be used for prolonged firing. The Oerlikon already had a simple antiaircraft mount, whereas the Hispano did not even have a fixed receiver, being designed to be mounted directly to the engine of an aircraft. The wisdom of choosing the Oerlikon was borne out by British experience with a few Hispanos mounted on escort vessels after supplies of the Oerlikon from Switzerland were interrupted: The Hispanos were so unreliable that ship crews preferred not to fire the guns at all in hopes that German aircraft would not identify them as warships.

The Oerlikon was usually shipped in single free-swinging mounts with splinter shields. Fire control was initially based on simple optical sights, but the Mark 14 gyroscopic computing gun sight was developed early in the war. The gun crew typically consisted of a gunner, trunnion operator, range setter for the gyroscopic sight, and two loaders. As the war progressed, ships undergoing refit tended to ship additional guns wherever there was enough space on deck to bolt one down, limited only by magazine capacity and top weight. (The single free-swinging mount, gun, and shield weighed about 1700 lbs [770 kg]).

However, the single 20mm proved too light a weapon to fend off the kamikazes. One destroyer commander reported that "when the 20 mm. opens fire, it's time to hit the deck", while another report noted that "20-mm. fire was a signal to the engine room to shut down the blowers to keep the flash of the explosion from the suicide hit being drawn into the machinery spaces" (Friedman 2004). A dual mount was first tested in September 1944, and single mounts were beginning to be replaced by dual or even quadruple mounts by the end of the war. Most of the dual mounts were also free swinging, but a few dual mounts and the quadruple mounts were powered and were directed using a joystick and the Mark 51 director. The latter was a Mark 14 mounted on a "dummy gun".

The sustained rate of fire of most models was much reduced by the need to pause and reload the 60-round drum magazine. An experimental quad mount using belted ammunition would have had a sustained rate of fire of 150 rounds per minute, but was superseded by by twin 40mm Bofors mounts.

Photo Gallery

20mm crewed by African-American sailors


20mm magazine


20mm gun crew in training


Twin 20mm in ground antiaircraft role

U.S. Marine Corps

20mm pedestal gun mount

U.S. Navy


Bureau of Ordnance #75 (accessed 2013-2-8)

Frank (1990)

Friedman (2004)

Campbell (1985) (2012; accessed 2013-2-8)

Roscoe (1953)

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