the fuel capacity was increased. Some ISU-152s were equipped with even larger external fuel tanks, two tanks on the rear hull deck, in addition to the four external fuel tanks — each, maximum — or with two smaller additional external fuel tanks, on the hull rear. This option was probably available for the post-war ISU-152 variants. Between December 1943 and May 1945, 1,885 ISU-152s were built. Mass production ceased in 1947, with 3,242 vehicles produced in total. Post-war ISU-152 modernisation included installation of night vision sights, replacing of the V-2IS engine with the V-54K, the 12.7mm machine gun was replaced by a newer variant, the ammunition capacity increased to 30 rounds, additional armor, automotive improvements and significant increase of the main fuel capacity. Variants The initial variant was developed in 1943. The factory designation was Object 241 (Объект 241). It was armed with the 152.4 mm ML-20S (МЛ-20С) model 1937 gun-howitzer, with a barrel length of over 4.2 metres (27.9 calibers). This gun had a maximum range of . The armor-piercing round, weighing , had a muzzle velocity of and a maximum penetration of of rolled homogeneous armour (RHA) at 90° at a range of . The rate of fire was 2-3 rounds/min. The ISU-152 carried 21 rounds of two piece (shell and charge) armor-piercing and high explosive ammunition. Later ISU-152 versions had a newer gun with a slightly longer barrel, up to over 4.9 metres (32.3 calibers), with a maximum range of fire of up to . The ISU-152 had different versions, with changes in the gun (later versions), the number of hatches, or the hull (based on the hull of the IS-1, IS-2 or IS-2 model 1944 tanks). The IS-2 based versions had a thicker gun shield and larger fuel tank. Until May 1944, the main armament was the 152.4 mm ML-20 model 1937 gun-howitzer. The ISU-152 had a rate of fire of 2-3 rounds/min. The early versions had three hatches in the superstructure roof and one emergency hatch at the bottom of the hull behind the driver's seat, which had an armored cover. Later versions had a fourth, round hatch in the superstructure roof on the right, next to the rectangular hatch on the left. ISU-152-2 ISU-152BM (ИСУ-152БМ), sometimes referred to as ISU-152BM-1 or ISU-152-1, with the factory designation Object 246 (Объект 246), was a single prototype developed in April 1944 in Factory No. 100 in attempt