Type 100 submachine gun The was a Japan ese submachine gun used during World War II , and the only submachine gun produced by Japan in any quantity . It was made in two basic variants referred to by American and British observers as the Type 100 / 40 and the Type 100 / 44 , the latter also known as the Type 100 ( Simplified ) . A third variant was a folding version of the early model , sometimes referred to as the Type 100 Navy , made for parachutists . Design . Designed and built by the Nambu Arms Manufacturing Company under a low - priority military contract , the Type 100 was a submachine gun that was first delivered to the Imperial Army in 1942 . Japan was surprisingly late to introduce the sub - machine gun to its armed forces — a few models of the SIG Bergmann 1920 ( a licensed version of the German MP 18 ) were purchased from Switzerland in the 1920s . These were examined and copied , with significant changes . In 1942 the Type 100 SMG was first delivered for service , used by Japanese marines during the invasion of Southern China . The Type 100 was typical of the class of simple , inexpensive , wartime submachine guns produced by all military powers — designed for maximum ease of production . It is based on a simplified Bergmann MP18 , modified for the 8mm Nambu round . It was an automatic - only , air - cooled , blowback weapon firing from an open bolt and feeding from a side - mounted , 30 - round detachable box magazine . The barrel was given six - groove , right - hand - twist rifling . Unusually for a submachine gun ( but typical of Japanese weapons of the era ) , a bayonet lug was fixed under the barrel , in this case with a heavy bar and lug . Some of these models featured a bipod , and others featured a complicated muzzle brake . The Type 100 had a chrome - plated bore to help fight corrosion in Asian jungle conditions . Its complex ammunition feed included a feature whereby the firing pin would not operate until the round was fully chambered ; frequent stoppages in firing were experienced in the field . The round was the underpowered and relatively ineffective 8 mm pistol round . The curved box magazine extending from the left side made for poor weapon balance when full . The sights were canted to the left . Versions . Two basic performance variants of the Type 100 were produced during the course of the war : the Type 100 / 40 was an early version with bipod and heavy bayonet lug , judged unsatisfactory because of frequent jamming of the feed mechanism , and the Type 100 / 44 was a simplified 1944 version that had a higher rate of fire and much greater reliability . A third variant was a lightened version of the early Type 100 / 40 design which was delivered with a folding stock for navy paratroopers . The Type 100 / 40 was complex , designed with little consideration for mass production . Its sights and feed mechanism were overly complicated . Its rate of fire was some 350 to 450 rounds per minute , and it jammed frequently . The 8x22mm Nambu pistol cartridge it fired was rather underpowered , compared to contemporary military pistol cartridges such as 9mm parabellum and . 45 ACP . In practice , the bayonet was not widely used . Some 10 , 000 were produced by the Kokura Arsenal for the Imperial Japanese Army . A folding version of this model was made for paratroops , designed with a folding stock for lighter weight . The reliability was not improved , and the folding stock proved less suited to close combat than the solid stock which stood up better in a buttstroke . Some 6 , 000 to 7 , 500 were made at Nagoya Arsenal and delivered primarily to the Imperial Japanese Navy for its marine paratroops ( " Rikusentai " ) , but also to Army paratroops — " Teishin Shudan " — who used them in February 1942 , during the Battle of Palembang , on raids against oilfields . Navy paratroops used the weapon in the first months of 1942 during the Battle of Manado and the Battle of Timor . The late war variant Type 100 / 44 , the simplified model , was designed in answer to suggestions coming from field units , and to hasten production at a time when Japan was being pushed into retreat across the Pacific Theater of Operations — demand for submachine guns was at an all - time high . The 1944 variant was slightly longer , with simple iron sights and a greatly simplified muzzle brake consisting of two ports drilled in the barrel . The bipod and the large bayonet mounting bar were eliminated , with the bayonet fitted to the barrel instead ; consequently , the muzzle protruded more from its perforated jacket . Corners were cut in production , leaving many Type 100s with roughly finished stocks and poorly welded parts . The relatively weak 8 mm round remained the same , but the rate of fire was significantly increased , to 800 rpm . The resulting Type 100 / 44 was quite light , with low recoil , and demonstrated good reliability and satisfactory accuracy for close - range work . Some 8 , 000 were made by Nagoya Arsenal . Despite these simplifications , Japan lacked the industrial infrastructure to produce suitable quantities of the Type 100 . By 1945 , only 24 , 000 to 27 , 000 had been built .