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. Type 100 on display at Battery Randolf US Army Museum, Honolulu.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.
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 quarters combat than the solid stock which stood up better in a buttstroke. Some 6,000 to 7,500 thousand 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.