The Japanese Model 98 (1938) 20-mm AA/AT machine cannon is a weapon
of unusual appearance, but with many practical advantages. The following account
of the gun has been taken from Special Series No. 18, "Japanese Infantry
Weapons," which is in process of preparation by the Military Intelligence Division.
It is an all-purpose weapon. Because of its lightness in weight and
maneuverability, it is an excellent gun for defense against low-flying bombers
as well as against ground-strafing fighters. An experienced gun crew can probably
place the piece in battery, ready for antiaircraft fire, in less than 3 minutes. In
an emergency, the weapon could be fired as a straight artillery piece, because of
its split trail and wooden wheels (see sketch).
The Model 98 20-mm AA/AT machine cannon may be identified by its
general appearance. It is different from any other Japanese field piece, the front
outrigger, the barrel traveling-lock and the marking which is on the top of the
receiver are distinctive features.
(This marking, which is read "Kyuhachi Shiki" from top to bottom, means "98 Model.")
(1) General. It is automatic and gas-operated. The cycle of operation is
loading, locking, firing, unlocking, extracting and ejecting; unlocking, extracting, and
ejecting are done during recoil; loading, locking, and firing in counter-recoil.
The recoil mechanism consists of two spring-loaded cylinders that lie one
on each side of the barrel. Air valves, located in the forward ends of the
cylinders, allow air to be drawn into the cylinders during recoil; as the air
cannot readily escape, it acts as a cushion during counter-recoil.
There is a vertical box-type magazine--with a capacity of 20 rounds--which
fits into a slot in the top of the receiver and is held in place by a spring catch.
For traveling, towing shafts are inserted in slots at the end of the trails, and
a traveling lock, which connects the forward part of the barrel to the bottom
carriage, holds the gun steady in transit.
|Principle of operation|| ||gas-operated, automatic or single shot|
|Ammunition||high explosive, tracer, and armor-piercing|
|Type of feed||20-round magazine |
|Length of barrel:|
| With muzzle brake||57 1/2 in|
| Without muzzle brake||55 1/8 in |
|Over-all length of gun with trails folded||180 in |
|Total weight of gun without wheels||836 lbs|
|Maximum horizontal range||5,450 yds|
|Maximum vertical range||12,000 ft|
|Muzzle velocity||2,720 f/s|
|Traverse without wheels||360°|
|Minimum elevation (depression)||-10°|
|Rate of fire||120 rpm|
|Length of recoil||adjustable from 2 in to 2.5 in|
(1) Safety. There are two safety features; (a) the lock on the firing
handle, to the left and rear of the gun, must be depressed before the handle can
be moved forward; and (b) the manual safety mechanism, to the rear and upper
right side of the receiver must be turned counterclockwise before the weapon can be fired.
(2) To place the gun in firing position: Set trails and outrigger in the
ground. Swing the eccentric (crankshaped) axle so that the weapon is resting on
the trails and outrigger, and the wheels are clear of the ground. Then the wheels
can be removed by releasing the spring catches, which lock them to the axle.
(3) The elevating handwheel is at the left rear.
(4) To traverse. Traversing can be done by the gunner by putting his
shoulder to the shoulder rest at the left rear of the weapon.
(5) To load. Place a loaded magazine into the slot on top of the receiver. The
first round can be pushed into the chamber by pulling the handcharging lever
into the rear and then forward. (The handcharging lever is at the right of the
(6) To fire. Press the lock on the firing handle and move the firing handle
forward. Automatic or single-shot fire may be chosen by moving the selector
switch at the right rear of the sleigh.
(7) To unload. If there is a misfired round in the chamber, pull back the
hand-charging lever in order to extract and eject the round.
There are two types of ammunition, high-explosive and armor-piercing.
(1) The high-explosive ammunition has an abnormally large brass cartridge
case, and a black projectile body. A green and yellow band are painted
together around the middle of the shell, and a red band just below the bourrelet.
The complete round weighs 14.5 ounces, and is 8.187 inches long. The
brass nose fuze is point-detonating, bore-safe, and supersensitive. There is a
tracer compound in the base of the shell.
(2) The complete armor-piercing round is 8.183 inches long. Both the
high-explosive and the armor-piercing rounds are wrapped individually in
cardboard containers. The armor-piercing ammunition is packed 70 rounds to a
The fuzes for the high-explosive round are wax-dipped, covered with a
metal cap, and paper-wrapped. The wrapped fuzes are packed between holes in
boards mounted in an unlined crate, and shipped separate from the rest of the round.
(1) Oiling and cleaning. This gun should be given the same care as U.S. machine
guns and automatic cannon. The bolt parts may be oiled lightly, except
in dusty or sandy countries, and then not at all.
(2) Stripping: (a) The barrel is keyed to the receiver by a set screw. When
this set screw is loosened, the barrel may be freed by releasing a latch on
the rim of the barrel bushing, and turning the barrel about one-sixth of a turn. The
gas cylinder yokes will come out with the barrel as one assembly. (b) Place the
bolt in the forward position with the handcharging lever. Remove the receiver
rear-plate assembly and slide the bolt out to the rear.
There is a carrying case for the sight, also a small box containing a
quantity of headspace washers. The ammunition is carried in a caisson which
may be horse-drawn.
The sight-mount carrying case contains the following items: main part of
apparatus, glasses, colored glasses, lighting apparatus, rain cover, small camel-hair
cleaning brush, towel, oil-can and three brushes.