Soviet troops advancing somewhere at Finnish front. June 1944.

Artillery

In April 1944, in attempt to increase the firepower of ISU-152, a high-power variant of the self-propelled gun was developed in Factory No. 100, designated ISU-152BM (ИСУ-152БМ), sometimes referred to as ISU-152BM-1 or ISU-152-1. The factory designation was Object 246 (Объект 246). The "BM" ("БМ") in the designation stands for "Large Power" ("Большой Мощности"). The main purpose of the ISU-152BM was the 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 barrel gun, which unlike the ISU-152's gun wasn't a gun-howitzer. The gun had a maximum range of 18,500 meters, with the 43.56 kg high-explosive shell which had a muzzle velocity of 880 m/s. The overall length of the gun was over 8 meters, with a barrel length of 7620 mm (50 calibers). The armor-piercing round, weighing 48.78 kg, had a muzzle velocity of 850 m/s. During test firing at armor plates with different thickness, the ISU-152BM have successfully penetrated a maximum of 203 mm of RHA at 90° at ranges of up to 2000 meters. However, during the trials, July 1944, the gun showed some deficiencies such as difficult to operate by the crew, unreliable work of the muzzle brake and the breech block, and unsatisfactory performance of the shells. In addition, the gun, reaching out too far, was limiting the maneuverability of the fighting vehicle. The self-propelled gun carried 21 rounds of two piece (shell and charge) ammunition, and had a rate of fire of 2 rounds/min. 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 barrel gun, with a slightly shorter barrel of 7392 mm (48.5 calibers). The self-propelled gun was designated ISU-152-2 (ИСУ-152-2). The factory designation was Object 247 (Объект 247). The fighting vehicle 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 18,000 meters, with the 43.56 kg 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 wasn't completed before the war ended. The fighting 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 19,500 meters, using a 48.5 kg high-explosive shell with a muzzle velocity of 880 m/s.

ISU-152-2 Prototype.

Soviet troops trying to hold ground against german forces during battle of Stalingrand in 1942