28mm type 89 grenade discharger

The Type 89 Grenade Discharger (八九式重擲弾筒, Hachikyū-shiki jū-tekidantō), inaccurately and colloquially known as a knee mortar by Allied forces, is a Japanese grenade launcher or light mortar that was widely used in the Pacific Theater of World War II. It got the nickname the "knee mortar" because of an erroneous Allied belief that these launchers could be fired by propping its plate against the leg. However, anyone trying to fire it this way would receive a severe bruise (or sometimes a broken thi