Following the capitulation of Japan on August 15, 1945, the first U.S. personnel flew into Atsugi Kaigun-hikōjō (Naval Air Facility Atsugi), Kanagawa Prefecture on August 28, 1945, starting the occupation of Japan that lasted until April 29, 1952. As was the case following the end of the war in Europe, the U.S. had a keen interest in gathering up any and all military technology that the Imperial Japanese Army and Imperial Japanese Navy had been using but more importantly, had been developing. Intelligence teams fanned out across Japan and one such group was deployed to the Tateyama Naval Gunnery School which was located in Tateyama, Chiba Prefecture.
Founded on June 1, 1941, the Tateyama Naval Gunnery School was a branch of the Yokosuka Naval Gunnery School. Whereas the latter school instructed pupils on operating and servicing guns utilized on naval ships, the Tateyama Naval Gunnery School taught students how to conduct land warfare, amphibious landing operations, and even airborne (paratrooper) training. As part of such training, anti-tank tactics had been included and as the U.S. intelligence team roamed the school, they found a most intriguing piece of anti-tank weaponry sitting on the grounds.
The Japanese, for a number of reasons, were very late in the game of developing potent tank destroyers. The main reason for this, and which also stymied tank development, was that early on, the Japanese rarely faced enemy armor that was either deployed effectively, employed in any real numbers, and in some cases, did not pose a serious threat. Other factors included not facing enemy forces that fielded much in the way of other anti-tank weaponry (such as anti-tank rifles and guns), jungle terrain that dominated much of the Japanese holdings did not favor armor, and since the Japanese relied on naval transports to send troops and equipment around the empire, this was a limitation in building heavier tanks. As 1943 wore on, the Japanese were encountering Allied tanks such as the M4 medium tank that proved quite a problem for Japanese tanks. Only the Type 1 47mm gun could offer some challenge to the M4 and this started to equip the Type 97 Shinhoto Chi-Ha (“New Turret” Chi-Ha) in 1942 but even then, the M4 was relatively immune to the Type 1 frontally. The Japanese responded with the Type 1 Gun Tank Ho-Ni I tank destroyer which was fitted with the more potent Type 90 75mm gun. Although production started in 1942, the Ho-Ni I did not see action until 1945. The successor, the Type 3 Ho-Ni III, was fitted with the same Type 3 75mm tank gun as used on the newer Type 3 Chi-Nu medium tank. Under forty examples were built starting in 1944 and were not deployed outside of the Japanese main islands. The Japanese had other tank destroyer designs, some of which made it to a prototype stage. The final tank destroyer design, the Type 5 Na-To, was to replace the Type 3 Ho-Ni III. Built upon the Type 4 Chi-So medium tracked carrier, the Na-To was fitted with a Type 5 75mm tank gun which was used on the Type 4 Chi-To medium tank. Given the state of Japan's industrial complex, only two Na-To were built and though 200 were ordered, the type never entered production.
What U.S. intelligence personnel saw at the Tateyama Naval Gunnery School is, to this day, debated on what exactly the vehicle was. Was it a tank destroyer prototype? Was it something that was created by personnel at the school as a ad-hoc vehicle to defend the facility? Was it simply a mobile test bed for the gun? What we do know is that the massive gun was the Type 10 120mm anti-aircraft gun. Developed in 1921, the gun entered service in 1926 and was fitted on Japanese aircraft carriers (namely the Akagi and Taiyo classes), cruisers (such as the Aoba, Yubari, and Myôkô classes), and other naval ships. The Type 10 was improved and remained in production from 1942 to 1945. The chassis the Type 10 was fitted to was the Type 97 Chi-Ha medium tank, one of the more common Japanese tanks of World War II.
The Type 10 had a weight of 2.9 tons, a uniform twist 34-groove barrel, with an overall gun length that was 18.3 feet long. The lightest single barrel gun mount, as used on some cruiser classes, was 7.7 tons. However, the gun was also used from fixed ground positions and the mount utilized was simpler and lighter than the ship mounts. The Type 10 fired two types of HE (High-Explosive) ammunition, an incendiary shrapnel round, and a illumination round among others. The ammunition used was fixed with a complete HE round being 3 feet long with an all up weight of 75 pounds. Muzzle velocity for the HE ammunition was between 2,707 to 2,723 feet per second and this gave a maximum range of 9.9 miles with the barrel at a 45 degree elevation. Other aspects of the Type 10 included a horizontal sliding-wedge, semi-automatic breech with a hydro-spring recoil system. A skilled gun crew was able to manage 10 to 11 rounds per minute but the average rate was more often 6 to 8 rounds per minute.
The Type 97 Shinhoto Chi-Ha had a weight of 16 tons total with a length of 18.1 feet. The image of the vehicle is a still from a film and is the only existing image available. It is a certainty that U.S. intelligence took photographs of the vehicle to document it but those photographs have yet to surface. From it, we can deduce several things. The first is the Type 10 is actually longer than the tank itself. The turret was, obviously, removed and the superstructure (upper hull) of the tank was cut away to reduce the height and provide a more flat firing platform. The gun is facing to the front of the tank and it is using a very cut-down version of the mount most often used when the gun was deployed from static positions. By consequence, the elevation of the Type 10 is severely limited though this isn't much of a concern if being used in the anti-tank role. The gun is showing some depression ability, likely no more than 10 degrees. There appears to be some form of crude gun shielding on the sides of the mount and on the front on either side of the gun tube. Other than this, the gun crew operated the weapon from the rear engine deck without any other protection from shrapnel or small arms fire let alone having no protection from the elements. There also appears to be a muzzle brake fitted to the end of the barrel as a means to reduce the recoil. The weight of the gun and gun mounting is evident by how compressed the suspension is on the tank. Given the size and weight of the ammunition, likely only a handful of rounds could be carried on the vehicle itself and assuming it entered service, either it was to be fitted with a trailer to carry additional ammunition or a support vehicle had to accompany the tank destroyer in the field to transport both the crew and ammunition. A anti-tank round was never developed for the Type 10 and the HE ammunition had between 3.75 to 4.1 pounds of explosive in the warhead. This was similar to the postwar Soviet 130mm KS-30 anti-aircraft gun, lighter than the German wartime 128mm Flak 40 which had a 7.5 pound explosive warhead, and also lighter than the U.S. 120mm M1 anti-aircraft gun. How the HE round would have been effective against Allied armor is unknown. Certainly a tank being hit by such a round would suffer ill effects. For example, the Soviet SU-122 assault gun utilized the 122mm M-30 howitzer that fired a HE shell with 8 pounds of explosive in the warhead and this was enough to demolish German tanks via concussive force upon a successful hit. Of course, how accurate the Type 10 would have been is debatable and it is doubtful it would have enjoyed any sort of accuracy beyond 1 mile. Another issue that is brought up is exactly how long the Type 97 chassis would have bore up to the recoil forces of the Type 10 before stress fractures appeared in the tank chassis. The additional weight of the gun would certainly have taxed the 170hp Mitsubishi SA12200VD diesel engine, reducing the 24mph speed of the standard Type 97 tank which would make “shoot and scoot” tactics difficult. In addition, the hull armor of the Type 97 was 25mm in the front, 26mm on the sides, and 20mm in the rear which was totally inadequate against any Allied tank gun. This would have forced the crew to fire at range, with the likely inherent inaccuracy, in order to be able to relocate before return fire is received.
To this day, it remains unknown what happened to the prototype following the discovery of it and, as already stated, exactly what its purpose was also remains a mystery.
Primary Sources:
DiGiulian, T. (2020, October 9). 12 cm/45 10th Year Type. Navweaps. Retrieved December 19, 2021, from http://www.navweaps.com/Weapons/WNJAP_47-45_10ns.php
Johnson, Curt Artillery (London: Octopus Books, 1976)
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