Type 91 grenade The 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 Although superseded as a hand - thrown weapon by the Type 97 by the start of World War II it was still used by units in the Second Sino - Japanese War and by reserve forces , as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 . 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 or discharged by a lightweight mortar - like projector ( the Type 89 grenade discharger , or " knee mortar " ) . As the Type 91 grenade was fitted with a time fuse , it did not explode upon contact . Instead , a weak creep spring inside the grenade firing mechanism caused the firing pin to be thrown back upon launching , igniting the time fuse . Using this system , Japanese rifle grenades could be launched right through jungle cover or through small openings without the danger of premature detonation in the event the grenade struck an object on its way to the target . The Japanese Type 100 Grenade Launcher was introduced in 1939 as a grenade discharger on Arisaka Rifles for Type 91 and Type 99 hand - grenades . The launcher is somewhat unusual in that rather than using the more common cup designs it is a gas trap system , meaning that it incorporates a barrel extension which taps off excess propellent gases to launch the grenade from a cup offset from the barrel . This has the advantage that standard rifle cartridges could be used along with the standard hand - grenades which simplified logistics , at the expense of increased weight and decreased efficiency . 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 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 . When used as a rifle grenade 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 floorboard 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 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 infantry men in the Second Sino - Japanese War and throughout the various campaigns of World War II .