20 mm Aircraft Automatic Cannon, Model 99 Mk 1 Flexible

20 mm Aircraft Automatic Cannon, Model 99 Mk 1 Flexible

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. The above illustration shows the flexible version.

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.


Caliber         20 mm—0.87 ins.
Weight (without magazine) 62 lbs.
Weight of 60 rd. magaine (empty) 20 lbs.
Length (overall) 55 ins.
Length of barrel 30 ins.
No. of grooves 9; Uniform right hand twist
Width of grooves
Depth of grooves 0.022 in.
Width of lands
Muzzle velocity (shell) 1,930 f/s.
Cyclic rate 510 r.p.m.
Traverse Flexible aircraft
Length of recoil
Turns of cocking handle required to cock piece 11 1/2 ins.
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

Japanese: p. 251 (June 1, 1945)

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