Type 91 grenade
The Type 91 hand grenade 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 Although superseded as a hand-thrown weapon by the Type 97 by the start of World War II it was still used by units in the Second Sino-Japanese War and by reserve forces, as well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. 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. The Japanese Army continued to experiment with rifle and hand-thrown grenades between the wars and would adopt a family of fragmentation grenades with almost universal adaptability. Introduced in 1931, the Type 91 fragmentation grenade could be thrown by hand, fired from a cup-type grenade launcher (the Type 100), discharged by a lightweight mortar-like projector (the Type 89 grenade discharger, or knee mortar). or fitted with finned tail-assembly and fired from a spigot-type rifle grenade launcher. The design of the Type 91 grenade was almost identical to the earlier Type 10. The main difference was the Type 91's domed 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 Type 89 grenade discharger. 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.