second, panoramic sight. The ISU-122 had a five-man crew; their location and roles were identical to those of ISU-152 crewmen. The ISU-122S was used with either four- or five-man crews since the semi-automatic breech allowed for a reasonable fire rate with a crew of four; the place of the absent loader was often taken up with additional ammunition. With a five-man crew (two loaders), the ISU-122S demonstrated a better fire rate than with four men. Organisation The organisation of military units utilizing the ISU-122 was the same as with the ISU-152. Soviet Army commanders tried not to mix the ISU-122 and the ISU-152 in one regiment or brigade, although there were some units equipped with both types. The different armament of the ISU-122 and the ISU-152 caused problems with ammunition supplies and logistics. Another disadvantage of mixed-type units was a doubling of the calculations required for massed indirect fire. Combat history The ISU-122 was used as a powerful assault gun, a self-propelled howitzer, and a long-range tank destroyer, the same as with the SU-152 and ISU-152 heavy self-propelled guns in general. However, these vehicles differed in their combat use. The primary application of the ISU-122 was as a tank destroyer, while the SU/ISU-152 tended more towards the assault gun role. With the same hull as the IS-2, the ISU-122 had good armour performance but very good performance with high-explosive rounds. The 122-mm gun had great potential, although the gun's abilities were somewhat reduced by the available projectiles and its lack of accuracy. In 1944, the BR-471 was the sole armour-piercing round available. An improved version, the BR-471B (БР-471Б), was developed in early 1945, but was available in quantity only after World War II ended. In extremis, the ISU-122 engaged enemy heavy armour with OF-471 (ОФ-471) high explosive projectiles. These shells had a mass of 25 kg, a muzzle velocity of 800 m/s, and were equipped with a 3 kg TNT charge. Mechanical shock and explosion was often enough to knock out enemy AFVs without any armour penetration. For urban combat, ISU-122s were utilized as assault guns, but with a lower efficiency in comparison to the SU/ISU-152. In general, Red Army commanders viewed the ISU-122 as a good assault gun. The OF-471 projectile was powerful enough against unprotected and entrenched infantry, pillboxes, and fortified buildings. In urban combat, the long barrel of the 122 mm cannon sometimes made maneuvering difficult. Use