German A. Korobov, Russian gun designer from Tula, began the development of assault rifles soon after the World War Two, when he designed the TKB-408 bullpup rifle for 1946-47 Soviet Army trials. Despite the failure of TKB-408, Korobov continued the development of various assault rifles, both in bullpup and traditional configurations. During late 1940s, he tried gas delayed blowback action in his series of TKB-454 experimental assault rifles, all chambered for standard issue 7.62x39 ammunition. While these rifles displayed some good results in accuracy department, these also showed insufficient reliability. By the 1952, Korobov switched to the Kiraly-type retarded blowback action, with the two-part bolt that uses braking action of the lever, interposed between bolt parts and receiver. This action allowed for significant increase of accuracy, as well as simplification of design and production, compared to then-standard Kalashnikov AK assault rifles.
During mid-1950s, Soviet Army initiates new trials for improved assault rifle design in the same 7.62x39 M43 caliber. Korobov submits his improved TKB-517 rifle, still based on the Kiraly type delayed blowback action; this weapon was extensively tested against modified Kalashnikov AK rifle, as well a number of other designs, and found to be superior to all. Korobov was found to be most accurate and controllable in full automatic mode (primary mode of fire, according to Soviet tactical doctrine), especially when fired from the shoulder or from the hip. It was also significantly lighter and less expensive to make than modified AK. Nevertheless, Soviet Army preferred less effective, but familiar and already well established Kalashnikov AKM over the more effective and lighter, but entirely new design.
TKB-517 is delayed (retarded) blowback operated weapon, that uses two-part bolt system, designed prior to WW2 by Paul Kiraly of Hungary. In this system, bolt has two parts - lighter breechblock with breechface and extractor, and heavier bolt carrier. A two-arm lever is interposed between these two parts; lower arm of the lever rests against the receiver when bolt is fully closed. When gun is fired, pressure in the chamber forces the cartridge case backwards and against the breechface. Bolt begins to travel back, but the lever acts as a mechanical disadvantage, transferring the short movement of the light bolt to the longer movement of the heavy bolt carrier. This action is sufficient to slow down initial movement of the breechface before the bullet leaves the barrel. Once the pressure in the barrel is low enough, the lever breaks the contact with the receiver, and the rest of recoil cycle both bolt parts complete as a single unit. Similar system later has been used in the French FAMAS assault rifle. receiver of TKB-517 has been made from stamped steel, furniture was made from wood. Charging handle was attached to the bolt carrier at the right side. Safety / fire mode selector was located above the pistol grip, also at the right side of the gun. TKB-517 used standard AK/AKM type magazines, including large-capacity 40 and 75-round ones, developed for RPK light machine gun.
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