"Conclusions on the possibility of using a single piece 122 mm shell in the D-25 gun on IS-122 and ISU-122S guns.
Based on instructions from GBTU, the NIBT proving grounds tested the feasibility of using a one piece 122 mm shell for the D-25 and D-25S guns on the IS tank and ISU-122S SPG, and came to the following conclusions:
- Loading the gun:
- In the ISU-122S SPG, the gun can be loaded at any angle.
- In the IS-122 tank, the gun can be loaded at the following angles:
- Horizontal: 360 degrees
- Vertical: -3 to +10 degrees
It is impossible to load the gun when it is elevated more than 10 degrees due to insufficient space between the gun breech and turret ring.
- The loading of the D-25 gun with a single piece shell is inconvenient and difficult due to a large size of the shell (1210-1400 mm long), heavy mass (about 40 kg) and limited movement space for the loader. Loading in the SPG is easier than loading in the tank.
- The ammunition capacity on the tank is reduced to approximately 27 shells compared to the current 28. In the SPG, the ammunition capacity is increased to 34 shells compared to the current 30.
Based on these results, GBTU considers it unreasonable to switch to a single piece shell for the 122 mm D-25 gun in the IS-122 tank and ISU-122S SPG for the following reasons:
- The rate of fire is not increased, as it is more difficult to load a single piece shell. It is impossible to load the tank gun at an angle of elevation of 10 degrees.
- The ammunition capacity of the SPG increases by only 4 shells, and the ammunition capacity of the tank decreases by 1 shell."