the chassis of the Kirovets-1, the initial prototype of the project that became the IS-3. The overall height of the vehicle was reduced to , which was compensated with an increased width of the superstructure.It was armed with the 152.4 mm ML-20SM model 1944 (МЛ-20СМ обр. 1944 г.) gun-howitzer, with a barrel length of over (29.6 calibers) and no muzzle brake. It had a maximum range of . The self-propelled gun carried 20 rounds of two-piece (shell and charge) armour-piercing and high-explosive ammunition. The armour-piercing round, weighing , had a muzzle velocity of . The rate of fire was 1-2 round/min. Object 704 had four hatches at the superstructure roof and one emergency hatch at the bottom of the hull behind the driver's seat, which had an armoured cover. The self-propelled gun carried two external fuel tanks, not connected to the supply system. The secondary armament of the fighting vehicle consisted of two 12.7×108 mm DShK machine guns, one anti-aircraft and one co-axial. The protection was increased by placing thicker armour at more radical angles. In the area of the gun, where the mantlet combined with the hull front behind it and the housing of the recoil mechanism, the armour thickness was . Object 704 was the best protected of all experimental or production Soviet self-propelled guns of the Second World War. However, the radical incline of the superstructure walls combined with the increased recoil of the gun, due to the lack of a muzzle brake, significantly complicated the work of the crew, and for this reason it was not adopted. ISU-152K Object 241K (Объект 241К) was the factory designation for a modernised variant of the wartime ISU-152, developed in 1956. It used a new engine, that of the T-54, with a cooling system and a heater. The capacity of the main internal fuel tank was increased to , which added more to the vehicle′s range on a road. The ammunition capacity was increased to 30 rounds after the removal of an additional internal fuel tank placed in the crew compartment. The gun had a maximum range of . It received a new commander's cupola, and also new sights. The running gear used many elements of the T-10. The mantlet had additional armor ring protecting the sight. Some of the ISU-152Ks received an additional 15 mm armour plate welded on top of the 60 mm armour plate covering the mantlet