Construction and Design
Construction of the ISU-122 and ISU-152 is the same except for the gun mounting, sights and ammunition stowage. The A-19S or D-25S cannons of the ISU-122 had 18 degrees elevation angle and 30 rounds of ammunition (the ISU-152 had 20 degrees and 20 rounds respectively). The A-19S cannon was equipped with an ST-18 (СТ-18) telescopic sight and the D-25S cannon was equipped with a TSh-17 (ТШ-17) sight. Both types of sights had maximal exact targeting distance of 1.5 km (the ISU-152 could manage only 900 m). Maximum direct fire range of the A-19S or D-25S cannons was 5 km, much further than these sights' targeting abilities. For direct or indirect firing on distances over 1.5 km the gunner used the second, panoramic sight.
The crew of the ISU-122 was 5 men and their location and roles were identical to those of ISU-152 crewmen. The ISU-122S was used with either 4 or 5-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.
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