The project SM-4 was supposed not only to achieve high combat performance, but also reduce the price of the finished fighting machine. It was therefore decided to make self-propelled mortar on the base of the truck Unimog U1700L. Equipment for the base chassis created a special platform, which carries all the elements of an artillery unit, and hydraulic outriggers. Last used to stabilize the war machine during the shooting. were mounted on the platform design of the original gun carriage, driven by hydraulic cylinders. The system of beams, rods and cylinders allow for a little time multicore lead to firing mortar or stowed position. To do this, hydraulics lowered the barrel breech block on the ground or raised up on a platform. To transfer the recoil earth block trunks equipped base plate area of 2 square meters. m with a strong opener. Retention mechanisms allow the mortar to turn a block of trunks in a horizontal plane within the sector width of 28 °. The elevation was regulated from 45 ° to 79
Combat Vehicle NORICUM SM-4 was carrying just four mortar barrel 120 mm, not having rifling. In anticipation of export orders software company to ensure compatibility with all relevant mortar ammunition manufactured to NATO standards. In the breech of the four barrels placed hammer mechanism with a pneumatic actuator. In this case, the breech had no means for charging as multicore carried muzzle-loading mortar. In the future, this feature has become one of the factors that have shaped the destiny of the whole project. In the mid-eighties he built the first prototype of the self-propelled mortar SM-4 on the basis of wheeled chassis Unimog. Combat vehicle with a total mass of about 7 tons showed very good characteristics of mobility and firepower. Austrian designers managed to make a relatively light artillery system, the dimensions and the weight of which completely fit into the capabilities of the underlying chassis. In addition, the war machine can be transported military transport aircraft C-130 and CH-47 helicopters (on the external load.) Test firing confirmed the declared characteristics. Four barrel length of 25 calibres enables you to shoot mine with an initial rate of about 510-520 m / s. During the tests was found maximum range - 11.5 km. Depending on the task mortar SM-4 can fully justify its name and shoot a volley of two or four shots (four shots took about one second). In addition, it was possible to fire a series, with a short interval between shots. The pneumatic hammer mechanism and fire control system allowed the calculation of choosing the most suitable mode of fire. Ammunition war machine was 60 minutes. In the calculation of the mortar were three people.
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