The Type 91 was hand-thrown infantry-level hand grenade utilized by the Japanese military throughout the 1930s and 1940s. It also saw widespread service in other regional conflicts thereafter. The series remained in active service with both the Imperial Japanese Army (IJA) and Navy (IJN) forces from 1931 until the end of World War 2 in 1945. Beyond this, it saw combat during the Soviet-Japanese Border Conflicts (1932-1939), the Second Sino-Japanese War (1937-1945), the First Indochina War (1946-1954), and the Korean War (1950-1953).
Design work occurred in 1931 and the Type 91 was developed as an improvement over the earlier Type 10 series of 1921 (detailed elsewhere on this site). Key areas of concern were damage potential and safety - qualities lacking in the Type 10 after some field experience could be recorded.
The Type 91's design included a safety cover, firing-pin holder, and firing pin at its slim, cylindrical top section. The percussion cap was situated beneath it near the "neck" of the arrangement. The main, segmented body of the grenade contained the bursting charge, detonator component, and a perforated steel disk. An optional attachment to the underside of the grenade's body was a cylindrical component housing the propellant and another percussion cap to allow the grenade to be "fired" from a weapon. Internally, the grenade has a basic filling of 65 grams of TNT. Overall weight reached 530 grams.
The Type 91 had the unique, inherent ability to be thrown by infantry, fired via rifle, or launched through use of the Type 89 "grenade discharger". As a hand-thrown weapon, the grenade held a delay of about seven to eight seconds before pyrotechnic detonation occurred. Range was only as far as the infantryman could throw it and the blast radius could cover poor accuracy to an extent. If fired from a service rifle, the Type 100 "cup" type assembly would be used to provided actuation with additional range and accuracy through direct line-of-sight engagement. When fired from the Type 89 grenade discharger (detailed elsewhere on this site), the weapon would be used as more of a lightweight infantry mortar with indirect line-of-site fire being possible. Beyond these three capabilities, the Type 91 could also be arranged as a "booby-trap" weapon in key areas to surprise unsuspecting enemies - this sort of flexibility was by design.
After a period of time, the Type 91 was found to have too long a useful detonation delay and basic measures were enacted to reduce this to roughly four seconds. In this way, the grenade could not be picked up by the enemy and hurled back at the owner. With this failing in mind, the Type 97 hand grenade of 1937 was also fielded alongside the Type 91 series into the war's final days.