Type 91 grenade

The Type 91 Hand Grenade (九一式手榴弾) was an improved version of the Type 10 fragmentation hand grenade/rifle grenade of the Imperial Japanese Army. Although superseded as a hand-thrown weapon by the Type 97 by the start of World War II it was still used by units in the Second Sino-Japanese War and by reserve forces, as well as the Japanese Navy's Special Naval Landing Forces.

Background and development
The Japanese Army, noting that grenades were short-ranged weapons, began efforts to optimize these weapons for close-in infantry fighting. The first hand-thrown fragmentation grenade was the Type 10. Soon after introduction of the Type 10 grenade to front line combat troops, a number of issues arose. When hand-thrown, instability and inaccuracy of the fuse mechanism made the Type 10 almost as much of a menace to the thrower as to the recipient. Furthermore, the weapon was regarded as undersized, and lacked desired lethality. In 1931, the Army Technical Bureau developed an improved version intended to address these issues. After carefully studying employment of grenades and mortars on the battlefield, the Japanese Army developed a unified system of hand grenades, rifle grenades, and grenade/light mortar shell dischargers ideally suited to warfare in typical short-range combat environments such as urban, trench, and jungle warfare. As part of this concept, the Japanese Army had adopted by 1932 a set of fragmentation grenades with almost universal adaptibility. The Type 91 fragmentation grenade could be thrown by hand, fired from a rifle via a spigot-type launcher, or used in a mortar-like grenade discharger, the Type 89.

Design
The design of the Type 91 grenade was almost identical to the earlier Type 10. The main difference was the Type 91’s dome top as opposed to the Type 10's serrated top. As with the Type 10, a threaded socket in the bottom of the body allowed for the attachment of an auxiliary propellant canister for use in a grenade launcher, or a finned tail assembly for use as a rifle grenade, fired from a spigot-type launcher. The fuse was a percussion-activated delay type, initiated by pulling out a safety pin and striking the top of the cap. The grenade incorporated a 7-8 second delay before detonation. This feature was incorporated as part of the Type 91's other uses as a rifle grenade or as a shell fired from the Type 89 grenade discharger, as the long delay enabled longer time-in-flight to distant targets. When used as a rifle grenade with a tail assembly added or mortar round with a base containing a primer and propelling charge, the fuse activated automatically, as the plunger was pushed in against a weak creep spring by the force of the launch. Additionally, the Type 91 could be used as a booby trap by removing the safety pin and setting under a floor board or chair.

However, the Type 91, as well as other Japanese hand grenades suffered from faults in manufacturing and production of the fuse, grenade body, and explosive compound, resulting in inconsistent detonation, variable fuse burning times, and incomplete or variable fragmentation of the grenade body. During the war, these manufacturing issues remained unresolved.

Variants
As a hand-thrown grenade, the 7-8 second delay of the Type 91 proved too long in actual combat, enabling the enemy to pick up and throw the grenade back. To resolve this, the Type 97 fragmentation grenade was adopted for hand-thrown use. In addition to a four-second delay, the Type 97 had no provisions for attaching a tail assembly for rifle grenade use, or a propellant base for firing by grenade projector. These changes prevented accidental usage in the latter roles while simplifying production. When the Type 97 with its shortened delay came into production and was being delivered to front line combat units, the Japanese Army continued to use the older Type 91 grenades as hand-thrown weapons, in addition to rifle and grenade projector use. Many of the Type 91 grenades were modified by shortening the fuse to a four to five second delay, drilling out the base, and welding small extensions on the body, so that it could not fit into the Type 10 or Type 89 grenade discharger, and painting the bottom white. The resulting modified Type 91 was visually almost identical to the Type 97 except for its white base.

Combat record
The Type 91 (modified) was issued as a standard hand grenade to Japanese infantrymen in the Second Sino-Japanese War and throughout the various campaigns of World War II.