The Japanese Knee Mortar
By Cliff Carlisle
The WW2 Japanese 50mm Grenade Discharger was called a Knee Mortar by the US GIs due to it’s curved base. They thought that the curve was placed around the leg at the knee and fired that way. This caused numerous broken legs due to the recoil of the weapon. The curve was so that it could be used on the ground or on a round log as was used for field fortifications.
The Japanese military used the grenade discharger to provide firepower at ranges between those of the mortar & hand grenade. They were used in the close support role both during the advance & in defense. They furnished firepower that was heavy in comparison to their weight & mobility.
What was listed as the first grenade discharger adopted by the Japanese in the WW2 US manual “Japanese Mortars And Grenade Dischargers” has now been identified as a Chinese 50mm Type 27 (1937 in Chinese calendar). Captured Chinese dischargers were used by the Japanese in the Pacific causing the Technical Intelligence people to assume they were Japanese. The Type 27 was a simple tube with a bore of 50mm. The bottom had a chamber for the gas generated when the charge was fired. There was a simple flat circular gas port cap on the side of this chamber. The cap had 2 different sized holes in it. Rotating the cap gave the grenade 2 different ranges. The large hole was for short range & the smaller for longer. There was a lever type trigger mounted in the shaft below the chamber.
Type10 Discharger Type 89 Discharger
The 50mm Type 10 (1921) Grenade Discharger was the first one to have the curved base. This was a smooth bore weapon and had an adjustable gas port at the bottom of the barrel to control the range of the grenade. This discharger fired the Type 10 & later Type 91 grenades with the booster screwed into the base. It also fired pyrotechnic (flare) rounds. The range with the Type 91 HE grenade was 65 to 175 meters depending on the gas port setting.
Type 10 grenade Type 91 grenade with booster.
The 50mm Type 89 (1929) Grenade Discharger is the most common discharger used in WW2. It had a rifled bore to increase both the range & accuracy of the weapon.
Type 89 shell. Type 89 shell disassembled.
The Type 89 shell was designed with a copper rotating band. The base of the shell had numerous holes drilled behind the rotating band. A powder charge was placed in the copper can shown in the disassembled view. When the primer was struck & the powder was ignited the pressure, acting through these holes, forced the rotating band out into the rifling of the barrel. At the same time the gas was vented out the holes shown in the bottom of the base piece. This propelled the shell out of the weapon. The white & yellow bands are the early color code used by the Japanese. Sometime in 1942 it was changed to a central yellow band. The red band just below the fuze indicates that it has an explosive filling. This was used on both early & late color codes. Incendiary & phosphorus smoke grenades were also fired from the Type 89 discharger.
The Type 89 Grenade Discharger had a lever type trigger coming out the side of the shaft below the barrel. There was a knurled knob to adjust the range at the bottom of the housing the barrel screwed into. A groove was machined down the side of the barrel & this groove was filled with white paint. This was a crude aiming device. Line the white line with what you want to hit & then tip the discharger to what you think is a 45 degree angle. A model was captured on Attu, the Aleutian Islands, Alaska, that had a simple double bubble leveling device on the barrel. When both bubbles were centered, the discharger was being held straight up & down & was at a 45 degree angle. This was not found on most of the Type 89 Grenade Dischargers.
As mentioned before, the range was controlled by a knurled knob. This knob was geared to a screw that ran through the shaft below the barrel. As the knob was turned, the screw turned. The screw was threaded through the base so it went up and down as it turned. This raised & lowered the shell in the barrel. When fired at the lowest setting, there was a small area for the gas to go into & a longer distance for the shell to travel before it left the barrel. This increased the velocity & hence the range. When fired at it’s highest setting there was a very large area for the gas to go into & a very short distance for the shell to travel, hence a low velocity & short range.
As the shaft goes up & down, the trigger goes with it. On both sides of the shaft are marked the range of the shell at that setting. The markings are for the Type 91 grenade on the right & the Type 89 Shell on the left. If you are using a Type 89 shell and estimate that the target is 150 yards away, you turn the knob until the trigger is lined up with the number 150 on the left hand scale.
So by holding the discharger at a 45 degree angle & lining up the target with the white line they could adjust the point of impact on the target by turning the knurled knob to the range that they estimate the target to be at. If they over shoot or under shoot, they just adjusted the knob to change the range. The range scales on the shaft go from 120 meters to 650 meters for the Type 89 shell. As the Type 91 grenade does not engage the rifling in the barrel there is considerable gas lost around it when it is used. Consequently, the range scale for it is only 40 meters to 190 meters.
This discharger is marked inside the base with a character in a circle that I would guess is a unit marking. The number 9 below it would be the weapon number in the unit. The discharger was issued with a cloth carrying case that had a tool kit/spare parts pouch in a pocket. The pouch contains a wrench and 4 spare springs tied together with a piece of twine, 2 ratchet parts for the trigger, 2 firing pins & 4 screws. Both trigger parts are serial numbered to the discharger as is the pouch & carrying case.