to increase the firepower of the ISU-152. The "BM" ("БМ") in the designation stands for "High-Powered" ("Большой Мощности"). The main purpose of the ISU-152BM was to fight against heavily armored tank destroyers such as the Elefant and the Jagdtiger. It was armed with the 152.4 mm BL-8 (БЛ-8) long-barreled gun, which unlike the ISU-152's gun was not a gun-howitzer. The gun had a maximum range of , with the high-explosive shell which had a muzzle velocity of . The overall length of the gun was over , with a barrel length of (50 calibers). The armor-piercing round, weighing , had a muzzle velocity of . During test firing at armour plates with different thicknesses, the ISU-152BM successfully penetrated a maximum of of RHA at 90° at ranges of up to . However, during trials in July 1944, the gun showed some deficiencies, such as being difficult for the crew to operate, unreliability of the muzzle brake and the breech block, and unsatisfactory performance of the shells. In addition, the gun, protruding far forward of the hull front, limited the maneuverability of the vehicle. The self-propelled gun carried 21 rounds of two-piece (shell and charge) ammunition, and had a rate of fire of 2 rounds per minute. It used the engine, transmission, running gear, and electric equipment of the ISU-122. In August 1944 the BL-8 gun was replaced with the improved 152.4 mm BL-10 (БЛ-10) long-barreled gun, with a slightly shorter barrel of (48.5 calibers). This vehicle gun was designated ISU-152-2 (ИСУ-152-2). The factory designation was Object 247 (Объект 247). It was also equipped with external fuel tanks. The gun had a modified muzzle brake and a semi-automatic breech block. It had a rate of fire of 3 rounds/min. The BL-10 had a maximum range of , with the high-explosive shell. In December 1944 the ISU-152-2 underwent trials, revealing the barrel strength and the angle of horizontal guidance were unsatisfactory. The gun was sent for further improvement, but it was not completed before the war ended. The vehicle was never adopted. After the war, the final and most improved, third modification of ISU-152-2 was completed. The gun had a maximum range of , using a high-explosive shell with a muzzle velocity of . Object 704 Object 704 (Объект 704) was the factory designation for a prototype self propelled gun developed in 1945. Also known as the Kirovets-2, it was developed on