1. GENERAL

To date, two types of grenade dischargers, both 50-mm, have been used by the Japanese. One is known as Model 89, heavy grenade thrower, and the other as the "10-year" type. These have been erroneously referred to as "knee mortars." They have a small attached base plate, designed to rest on the ground or any solid object while firing--never on a soldier's knee or thigh. A Marine on Guadalcanal fired one of the dischargers from his thigh, and his upper leg bone was broken by the force of the recoil.

The Japanese are using both types of dischargers in the South Pacific fighting, and a number of them have been captured by our forces. Studies and experiments with both types have been made by the U.S. Ordnance Department, and the information in this section is based on the Ordnance findings. The dischargers were found to be very effective, easily carried, simple in design, and easy to manufacture. Designed for use by the individual soldier, they bridge the "gap" between hand grenades and regular mortar fire.

2. MODEL 89

a. Description

The Model 89 was perfected in 1929, and is considered to be an improvement over the earlier (1921) model.

All component parts of the Model 89 (see fig. 1) are made of steel. It is constructed in the form of a pipe-like, rifled barrel, which is attached to a small base plate. The plate is so shaped on the bottom that it could fit over a medium-sized tree trunk or log. The trigger housing is a tubular piece of metal with a lengthwise slot. Protruding through this slot, the trigger cocks and fires in one operation. A spring sleeve covers the trigger housing at the base plate. A range-adjusting assembly is attached to the base cup of the barrel.

b. Table of Characteristics

Caliber		50 mm (1.97 in)
Length (over-all)		24 in
Length of barrel		10 in
Weight		10 1/4 lbs
Range		65 to 700 yds
Weight of projectile		1 lb 12 oz
Barrel		Rifled

c. How It Operates

The discharger has range scales on both sides of the lengthwise trigger slot, and the weapon can be set at the desired range by turning the range-adjusting knob. When the knob is turned, it lengthens or shortens the worm housing inside the barrel of the discharger; Thus, the range of the weapon can be regulated by lengthening or decreasing the distance traveled by the projectile through the barrel. The weapon can be fired from the ground or any other solid base.

Figure 1. Japanese Model 89 Grenade Discharger and Ammunition. (The correct position for firing is shown in the bottom view.)

An angle of about 45 degrees is believed to be the most effective position of the discharger for firing. It has no sight except a grooved line which extends from the muzzle for a short distance down the barrel. Preparatory to operation, a grenade is placed in the barrel and allowed to drop to the firing housing, where it remains until fired. When the operator pulls the trigger (by pulling a short leather lanyard which is fastened to the trigger), the following action takes place: Cogs in the trigger move the firing-pin housing forward by engaging cogs in front of the housing; this causes the firing-pin spring to be compressed. As this action takes place, the tang on the cocking piece engages against the cocking lug of the firing-pin shaft. A continued pull on the trigger allows the cocking piece to become disengaged from the cocking lug, and the tension of the firing-pin spring, upon being released, sends the firing pin forward and sets off the propelling charge.

3. '10-YEAR' TYPE

a. Description

This discharger, also constructed entirely of steel, was perfected in 1921. It is still being used by the Japanese, mainly for firing signal pyrotechnics into the air. It also fires a fragmentation-type grenade.

The weapon is muzzle-loaded, and is fired by a striker which is operated by a Lever on the outside of the discharger body. Like the newer type, the 1921 model is fired while attached to a small base plate. No bipod attachments are used.

The discharger fires grenades that weigh nearly 1 pound. A special attachment containing the propelling charge and percussion cap is screwed into the grenade base.

b. Table of Characteristics

Caliber		50 mm (1.97 in)
Length (over-all)		20 in (about)
Length of barrel		9 1/2 in
Weight		5 1/4 lbs
Range		65 to 250 yds
Barrel		Smooth bore
Transport		Carried on the man
Weight of grenade		1 lb

c. How It Operates

The weapon is fired by a trigger that is attached to the bolt housing. When the trigger is pulled, the following action takes place:

The trigger pin rotates in a notch, and the trigger lips compress the firing spring by engaging and forcing the cocking-piece sleeve forward on the firing-spring guide.

When the sear has rotated approximately one-half the way back, it slips off the notch and releases the firing-spring guide, which jumps forward because the firing spring is compressed. Thus the firing pin contacts the primer. When the trigger is released, the firing spring--which still has a slight tension--forces the trigger back into position to be fired again.

Upon being released, the sear moves back over the notch, which has a small leaf spring that depresses and also allows the notch to move downward. This latter movement allows the sear to go back to its original position so that the piece can be fired again.

This complete operation takes place each time the trigger is pulled, since the firing mechanism is of the continuous-pull type.

The range-control gauge governs the range of the projectile by the opening or closing of a gas port in the base of the barrel. This gauge can decrease or increase the force of gas expansion in the barrel, thereby regulating the range. By opening the gas port to its fullest extent, the range is decreased to its minimum; by closing the port, the range is increased to its maximum. (The elevation also must be taken into consideration.)

4. AMMUNITION

a. For Model 89

Only the 50-mm high explosive type of grenade projectile was available to the Ordnance Department for examination. However, the Model 89 is believed to be designed to fire gas or smoke projectiles as well as high explosives.

The effective bursting radius of the high-explosive shell was found to be 30 feet. The explosion caused 211 impressions on a low-panel bursting range used by the Ordnance Department in conducting the tests.

The grenades received to date were painted with black enamel, with a 1/4-inch red stripe at the head and a 5/16-inch yellow band around the center of the grenade body. Each round was wrapped, unfuzed, in water-resistant paper with a plastic-closing plug. The fuze accompanying each round was wrapped in paper and excelsior and separately contained in a small tin can.

The projectile consists of three major parts, the fuze, the body, and the propelling charge. The fuze is of a simple point-detonating type with a pin safety. After the pin is pulled out, the fuze is armed by a setback and centrifugal force on firing from the discharger.

The body of the projectile is made of mild steel. It serves as a receptacle for .31 pound of TNT-type explosive filler.

The propelling charge consists of the percussion cap, propellant, and an expanding copper rotating band. This unit is assembled to the body by a screw thread, and is fired when a striker hits the percussion cap. When the firing action occurs, the expanding gases exert their force on the copper band and drive it against the rifling. This causes the projectile to rotate.

b. For "10-Year" Type

Only the high-explosive, fragmentation-type grenade projectile was available to the Ordnance Department for study in connection with the "10-year"-type discharger. This projectile is the standard Japanese hand grenade with a modification enabling a propelling charge to be added to it.

The range of this smooth-bore ammunition is reported to be 65 to 200 yards, depending on the adjustment of the gas port of the discharger. No determination of ballistic data could be made because of the lack of sufficient ammunition to conduct a firing program.

No distinctive markings were found on the grenade examined.

Like the projectile for the Model 89 discharger, the grenade for the 10-year type consists of three main parts, the fuze, the body, and the propelling charge. The fuze is of a simple firing-pin-initiated, powder-delay type with a pin safety. The delay is approximately 7.5 seconds. However, it is reported that when this fuze is used in the discharger, it will also detonate upon impact. To arm and use as a hand grenade, simply pull out the safety pin and hit the firing pin a smart blow. When fired from the discharger, the pin is pulled out, and it is reported that the fuze will arm on setback. Sufficient ammunition was not available to check this statement.

The body consists of a mild steel filler-cap plug and a cast-iron body proper. The body serves as a receptacle for approximately 3 ounces of what appears to be TNT explosive filler. To it is affixed the filler cap and propelling charge.

The propelling charge consists of a percussion cap and propellant. When the firing pin strikes the percussion cap, the propellant is ignited and fires the grenade.