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The Type 99 was the standard cannon on Japanese naval aircraft throughout the war, appearing in both fixed and flexible variants.


It was based on the Oerlikon, for which the Japanese had negotiated a manufacturing license in 1936, but it was continually improved and refined. Early marks were known simply as E-shiki ("Oerlikon Type") guns, and it was not until 1941 and 1942 that the Mark 1 and Mark 2 were so designated. As with all Oerlikon variants, the Type 99 action was blowback with advanced primer ignition.

The Type 99 was not an outstanding weapon. Although its APIB action made the Type 99 one of the lightest aircraft cannon of the Second World War, suitable for the equally light Japanese Navy airframes, this came at the cost of a low muzzle velocity and rate of fire. The Type 99 was inferior to the Japanese Army's excellent Ho-5 cannon, which used a short recoil action, in every respect except overall weight. However, with the A6M "Zero" under design at the time that the Type 99 was adopted, weight was the preeminent consideration. It did not help that there was practically no cooperation between the Army and Navy on weapons systems and that the manufacturer of the Type 99, the Tomioka Weapons Works, had been set up by retired Navy officers.

The Mark 1 was based on the Oerlikon FF and came in both fixed and flexible marks. Early models were fed from 60-round drums, but by 1942 this was replaced with a 100-round drum. The Mark 1 Model 4 was belt-fed and had a slightly faster rate of fire than the other marks. All marks had rather low muzzle velocities, which initially made them unpopular with aircrew.

The Mark 2 was based on the Oerlikon FFL and came into use as airframes became large and strong enough to accommodate the bigger, more powerful gun. However, the Mark 2 did not completely close the performance gap with the Ho-5. All models were fixed guns, though these were sometimes installed in mechanical turrets for use in flexible gun positions. As with the Mark 1, early marks used 60- or 100-round drums, while the Mark 2 Model 4 and Mark 2 Model 5 were belt-fed. The Model 5 introduced a number of innovations in the belt feed and bolt buffer to achieve a higher rate of fire, but it was not introduced until May 1945 and saw little service.

20 mm Aircraft Cannon Type 99 Mk. I[]

This weapon is an air-cooled, blowback-operated, Oerlikon type machine cannon. It operates on the same basic principle as all Oerlikon cannons of this type. The Japanese gun is a close copy of the Swiss gun, in that it is designed for full automatic fire only. The gun is manufactured in Japan on Swiss machinery.

A significant feature is that the parts which are subjected to little wear, such as the grips, mounts, gunners’ shoulder rest, and other exterior parts are generally made of light weight metal.

This weapon is almost identical with other Model 99 (1939), 20 mm aircraft cannon reported to be used in the majority of Japanese planes, both as fixed guns in fighter craft, and as flexible guns in bombing planes. The weapon is fed from a drum type magazine. It is cocked or charged by manual means, and has no semiautomatic charger or rounds counter. The cocking handle is rotated to draw the recoiling parts to the rear and cock the gun for the first shot, the gun firing from an open bolt. Cocking operations for succeeding shots are performed by the blowback operation of the gun itself.

20 mm Aircraft Cannon Type 99 Mk. II[]

This is a gun of higher power than the Type 99 Mk. I, 20 mm cannon. Like the earlier gun, it operates on the Oerlikon principle and is found both with drum type magazine feed (Mod. III) and with belt feed (Mod. IV).

The principal differences between this model and the Mk. I consist of a longer barrel and a longer chamber. The barrel protrudes 18 inches beyond the leading edge when mounted in the wings of fighter aircraft. The projectiles used are identical to the Mark I, but the cartridge employed contains approximately 40% more propellant than the older type, thereby increasing the velocity of the Mk. II 500 to 700 foot seconds. The muzzle velocity of the weapon varies from 2,500 to 2,700 foot seconds depending upon the type of projectile used. The gun has been found in ZEKEs and HAMPs (US codenames for variants of A6M Zero).

Design Details[]

Type 99 Mk. I

  • Caliber: 20 mm (0.87 in.).
  • Weight (without magazine): 62 lbs.
  • Weight of 60 rd. magaine (empty): 20 lbs.
  • Length (overall): 55 ins.
  • Length of barrel: 30 ins.
  • Muzzle velocity (shell): 1,930 f/s.
  • Cyclic rate: 510 rpm.
  • Ammunition: HE; HE with tracer; HE with self-destroying tracer; HE-I; AP; AP tracer; AP-HEI; Long burning tracer; Practice
  • Wt. of HE projectile: 4.50 ozs.
  • Type of feed: 60 rd. drum.

Type 99 Mk. II

  • Caliber: 20 mm (.787 in.)
  • Weight of gun: 67 lbs.
  • Length of gun (overall): 73 ins.
  • Length of barrel: 47 ins.
  • Length of rifling: 41.5 ins.
  • Principle of operation: Blow back
  • Feeding device: French drum or belt
  • Capacity of drum: 100 rds.
  • Cooling system: Air
  • Sights: Reflector type
  • Charging mechanism: Pneumatic
  • Firing system: Flexible cable
  • Effective range (est.): 600-700 yds.
  • Rate of fire (est.): 400-500 rpm.
  • Ammunition: Ball, A.P., A.P./I., T., H.E., H.E./T., H.E./I.



  • Type 99 Mk. I Model 1
  • Type 99 Mk. I Model 3
  • Type 99 Mk. I Model 4
  • Type 99 Mk. II Model 3
  • Type 99 Mk. II Model 4
  • Type 99 Mk. II Model 5 (Longer barrel)



  • Type 99 cannon
  • National Archives #80-G-193284
  • McAulay, Lex. 1991. Battle of the Bismarck Sea. St. Martin's Press. ISBN 0-312-05820-9.
  • Williams, A.G., and Gustin, E. 2003. Flying Guns: The development of aircraft guns, ammunition, and installations 1933-45. Wiltshire, England: Airlife Publishing. ISBN 978-1-84037-227-4.
  • Mikesh, Robert C. 2004. Japanese Aircraft Equipment 1940-1945. Schiffer Military History. ISBN 0-7643-2097-1.