And shot at. CAMD RF 38-11369-64 tells us why:
"Trials of the tank turret by shooting
The tank's armour was tested with mount testing shells, in order to find out the reliability of the optical sight and its mounts.
Factory #100 performed a shelling of the tank with armour piercing shells in order to determine the reliability of various components of the tank. At the same time, this was used by our commission to test the sight.
The IS turret had a new 4x sight installed and carefully calibrated.
The tank as shot at with a 76 mm ZiS-5 gun from a distance of 300 meters. 10 shots were fired, 5 shots with mount testing shells, and 5 shots with armour piercing shells.
The mount testing shells were aimed at the area of the sight, with two shells impacting the shell on the level of the sight, and three in the mantlet on the same plane.
The armour piercing shells were used to test the sturdiness of various parts of the tank (driver's hatch, commander's cupola, etc). The hits were as follows:
- Driver's hatch: one hit
- Front armour: two hits
- Commander's cupola: two hits"
Figure 7. Blue dots represent the mount testing shells. Red dots represent armour piercing shells.
"After shelling, the sight was examined externally and looked into. The result was as follows:
- The sight was free of deformations and defects.
- Sight adjustment and sharpness did not suffer.
- Calibration of the sight was not knocked off.
The support pole of the ocular portion of the sight, welded to the turret, fell off. The ocular part fell down, to the hydraulic turret traverse mechanism. This was a defect of the installation, and not the sight. Additionally, no defects or damages were caused by the falling of the sight.
These tests show that the PT8-15 sight is sufficiently rugged and continues to function after numerous nonpenetrating hits. The sight mount is adequate. The only part that needs perfecting is the support pole for the ocular part of the sight."
"Figure 8. KV-85 tank with IS turret and PT8-15 sight after shelling. The white marks on the turret are results of the mount testing shells."
"Mount testing shells" are a special kind of shell, used for training. It is a homogeneous shell, with the same mass as the shell it's supposed to represent, but without things like an explosive filler. Obviously, these shells have lesser penetration than regular AP, but the IS turret withstood both types of shell from 300 meters, something Tigers weren't quite capable of.