IS Gun Upgrades

At the first stages of IS tank development, three objects were tested: 237 (what ends up being the IS), 238 (KV-1S with the 85 mm S-31 gun) and 239 (KV-85 prototype). A number of requirements were drafted to improve the prototypes. Item 6 of the list was by far the most interesting.

CAMD RF 38-11355-1788

"Along with artillery engineering bureaus, develop:

1. A gun mount, and a turret, that can carry barrels of the following calibers: 85, 100, 122, 152. The loader is to be positioned on the left, the gunner and commander to the right.
2. Installation of a [illegible] optical sight, in conjunction with the MK-IV observation device.
3. A hydraulic gun stabilizer.
4. A breech-loaded 50 mm mortar for self-defense and flares.
5. A gun position indicator for the commander and gunner, indicating the gun's angle and quadrant."