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Mortars and grenade dischargers

The grenade discharger was designed for use by the individual soldier and served to extend the range of the hand grenade as an intermediary weapon approaching the true mortars. It had a curved baseplate which made it appear as though it could be fired while the weapon was resting on a part of the human body and, therefore, was frequently, but incorrectly, referred to as the "knee mortar." Actually, the baseplate was made to fit over a tree trunk or a log or to be stuck into soft earth. The weapon was never intended to be fired while resting against the thigh, as some gullible individuals discovered to their dismay.

The 50 mm. grenade discharger, Model 10 (1921), was a steel, smoothbore weapon with an overall length of 20 inches, a barrel length of 9? inches, and a total weight of about 5 pounds. The ammunition, a Model 91 hand grenade with safety pin removed or a pyrotechnic grenade, was inserted into the muzzle. Upon pulling an external trigger lever, the propellent train was ignited. The setback activated and armed the fuze. With the Model 91 hand grenade, the estimated range was from 65 to 250 yards.

In 1929, the Japanese perfected the 50 mm. Model 89 grenade discharger which was an improvement over the Model 10. The discharger had an overall length of 24 inches, the barrel measured 10 inches, and the total weight was 107 pounds. A distinguishing feature of the barrel was its rifling. A Model 89 HE (high explosive) shell was designed with a rotating band which expanded against the rifling. In addition, the Model 91 hand grenade could be used as ammunition. The Model 89 HE shell had a range of 131 to 710 yards, and the Model 91 hand grenade had a range of 44 to 208 yards.

Intermediate between the grenade dischargers and more conventional mortar designs was the 70 mm. mortar, Model 11 (1922). This weapon had a rifled tube and fired an HE projectile of the same design as that used in the Model 89 grenade discharger. The propellent charge was contained within the base of the projectile, and firing was accomplished by the impact of a percussion hammer against the firing pin. The weapon had an approximate range of 1,700 yards.

The most commonly encountered Japanese mortars were 81 mm. and 90 mm., and they were very similar in appearance to the U.S. 81 mm. mortar, M1. Among the 81 mm. mortars were two models, Model 97 (1937) and Model 99 (1939). The Model 97, 81 mm. mortar (fig. 5) was operated in the same manner as the U.S. 81 mm. mortar and used an HE shell, Model 100, weighing 7.52 pounds. The shell was also similar in appearance to the U.S. 81 mm. M43A1 mortar shell. Model 99 was a smoothbore mortar which weighed only 52 pounds but was found to fire a 7.2-pound shell approximately 2,200 yards. The projectile for Model 99 was again similar to the U.S. M43A1 ammunition, and the two forms were found to be interchangeable. One distinguishing feature of the Model 99 mortar was a movable firing pin which was brought into action by striking a firing-pin camshaft with a mallet.

Of the 90 mm. mortars, the prototype was the Model 94 (1934). This was a smoothbore, muzzle-loading weapon which was characterized by its heavy recoil mechanism. This mechanism furnished greater stability with higher powder pressures but increased the weight of the weapon to 353 pounds. The mortar had a fixed firing pin and was fired in the same manner as the U.S. 81 mm. mortar. Its HE rounds weighed 11.9 pounds. The approximate range was 4,050 yards. A Model 97, 90 mm. mortar (fig. 6) was issued in 1937 and had the same general appearance as the Model 97 (1937) 81 mm. mortar. It differed from the Model 94 mortar in the absence of the heavy recoil mechanism and tube reinforcing hoop and weighed 120 pounds less. Otherwise, it fired the same ammunition as the Model 94 and apparently had the same range. If instantaneous contact action was not required, a delay element could be placed in the nose of the fuze.

In addition to these commonly found mortars, the Japanese had others of conventional design in 120 and 150 mm. sizes with unconfirmed estimates of ranges as high as 5,000 yards. For sheer size, the Japanese had a 320 mm. spigot mortar which fired a 674-pound shell. Ammunition for a 250 mm. spigot mortar reportedly produced a radius of burst of 273 yards. In these models of the mortar, the Japanese principle of the heavy shell-that is, designing weapons to fire the largest possible shell from the lightest possible weapon-was expressed in its most extreme form.