Conservative elements within the tank corps favoured the Nagant M1895 revolver. It was relatively simple and could be fired out of a smaller pistol port than the TT. However, the Nagant took much longer to reload and could not use submachinegun ammunition in a pinch.
"For ISU-122 crews it is desirable to have a TT pistol, since the long loading time for Nagant revolvers puts the crew into a critical position. The limited amount of ammunition in a revolver causes frequent reloads. There is also much more ammunition for a pistol available inside the vehicle."
Via Andrei Ulanov.
