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Type 99 81 mm mortar
The Type 99 81 mm mortar (Japanese as "Kyukyu Shiki Shohakuyekiho", meaning "99 model small trench mortar") was a Japanese mortar used primary by Imperial Japanese Army during World War II. The Type 99 designation was given to this gun as it was accepted in the year 2599 of the Japanese calendar (1939).〔War Department Special Series No 30 ''Japanese Mortars and Grenade Dischargers'' 1945〕 The Type 99 81 mm mortar is typical of the Stokes-Brandt type mortar.〔The Stokes-Brandt mortar is muzzle-loaded and has a smooth bore. It fires a rocket-shaped projectile with tail fins, which is very stable in flight,with the nose of the shell kept forward throughout the trajectory. The fuze used is the point-detonating type.〕 The Type 99 81 mm mortar differs from the Type 97 81 mm infantry mortar in the shortness of its tube and in the method of firing. The differences between the Type 99 and the US 81-mm mortar, M1 are pronounced.〔War Department TM-E 30-480 ''Handbook on Japanese Military Forces'' 1 October 1944〕
==Design==
The Type 99 is a smooth bore, muzzle-loading weapon of the Stokes-Brandt type.〔〔 The Type 99 81 mm mortar differs from the Type 97 81 mm infantry mortar in the shortness of its tube, which is only 21.75 inches as compared with 45.34 inches of the Type 97. The mortar can be disassembled into three units: the tube, the bipod, and the base plate. The legs of the bipod, made of tubular steel, are mounted on the elevating screw housing by a clevis joint. They terminate in thin steel plates which have single-pointed spikes on the underside. The spread of the legs is limited by a chain which has a spring attached to one end to relieve the shock of firing. A buffer system incorporated in the bipod gives a recuperation of 2 inches. The recoil cylinders are filled with light grease or heavy oil. The base plate, made of pressed steel, has a series of ribs and braces welded on the underside to allow it to dig into the ground when the piece is fired. On the top side of the plate, in the center, is a circular depression into which the base-cap knob fits when the mortar is mounted in firing position. A carrying handle is attached to the rear edge of the plate. Also, two small hooks are welded to the rear edge to permit the attachment of a carrying strap. Around the outer edge of the plate is a one-inch ridge, and on the lower front edge a hook is attached through which a small rod can be thrust and used for slight changes in base plate alignment. A stability test of the Type 99 mortar, in which a total of 14 rounds were fired, produced no visible evidence of cracking or deformation of either base plate or bipod. The plate sank about 2 inches into the ground with the firing of seven rounds at alternate elevations of 45 to 75 degrees. Stability was very satisfactory when firing zone charges I to IV, but zones V to VI produced noticeable hop of the mortar and a sinking of the base plate. The reported range of the weapon of about 2,200 yards has not been conclusively confirmed.〔
The differences between the Type 99 and the US 81-mm mortar, M1 are more pronounced. The tube length of the Japanese weapon is only about half that of the US mortar 59.5 inches vs 25.5 inches. Of much greater significance, however, is the difference in the method of firing. Whereas the Type 97 has a fixed firing pin in the base cap, the Type 99 has its firing pin affixed to a camshaft that extends outside the base cap of the mortar. This shaft must be struck a sharp blow with a mallet to drive it inward so as to force the firing pin against the primer of the propellant cartridge. The primer ignites the propellant charge of the cartridge, which in turn ignites the powder increments attached to the fins. The weapon fires with an exceptionally loud report and a pronounced muzzle flash. When the firing pin camshaft is struck, the camshaft spring is extended. When it snaps back into its original shape, it withdraws the camshaft thus bringing the firing pin down into the base cap in position for the next shot.〔 In addition to the different methods of firing, the Japanese weapon also employs a turnbuckle for cross-leveling instead of an adjusting nut and connecting rod, and its recoil mechanism also differs from that of the US piece. Finally, the Japanese base plate has only one socket for the base cap knob, and it is not provided with a sighting line.〔
When in firing position, the mortar tube is attached to the bipod by a clamp. It then is fastened to the base plate by the insertion of the spherical knob on the base cap into the socket on the plate and rotating the mortar 90 degrees right or left. The tube is smooth bored, and its interior surface is carefully finished. Tolerance between the wall of the tube and the bourrelet of the shell is very close, thus preventing the shell from striking the firing pin with sufficient percussion to detonate the primer. On its outside surface the tube has a sighting line and quadrant seat at the muzzle end. The base cap of the tube is hollowed and threaded to screw onto the breech end of the tube which it thereby seals against gas leakage. The cap terminates in a spherical knob, which locks into the socket of the base plate and is bored and is threaded axially to receive the firing pin and the base cap plug.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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