Development of 85 mm tank guns in the USSR began on May 5th, 1943. Multiple competing variants were developed. A new tank was being designed in parallel, and when it was finished there were two 85 mm guns available for it: the S-31 and D-5.
The first gun was to be finished was the S-31 designed by the TsAKB. 4 of these guns were finished in late June of 1943 and two were installed in the Object 237, the future IS-1. This gun was built to the military's requirements and used many F-34 components to speed up production.
S-31 gun in the Object 237 turret.
S-31 gun in the Object 237 turret, view from above.
By tank standards, this is fairly roomy. The loader has a 690 mm wide workspace, larger than even the Tiger's loader had. The gunner has a lot of space to work in too, but the commander is a little cramped due to the amount of room the gun takes up. There are 300 mm between the breech and the turret ring, just enough to squeeze by.
The D-5T gun in the Object 237.
The D-5T gun in an Object 237, seen from above.
The D-5 gun built at factory #9 did not follow these requirements. As a result, even though the gun breech was a bit wider, it took space out of the gunner's side, not the loader's. The breech was also a lot shorter and left 450 mm of room behind the gun, just enough for the shoulder span of an average adult male. The commander would have no issues in his station or if he had to pass behind the breech to get to the loader's side. This gun also had shorter recoil (240-300 mm vs 540 mm) and the mantlet was 270 kg lighter. It also didn't need extra weights to balance the gun.