The Nagant revolver uses the 7.62x38R Nagant, it is a very low pressure round. Even though this revolver uses a unique pressure seal cylinder system, it is hardly up to par we the pressure developed by the 7.62x39mm. The Nagant brothers originally designed this revolver to use a black powder cartridge. Having latter included the gas seal cylinder system which further weakened the revolver and modified the round for the then new smokeless powder, a low pressure cartridge was called for to prevent the gun from blowing up. The 7.62x38R is ballistically comparable to the .32 ACP, you do the math.
Personally I would not fire the Nagant revolver with anything but it original low pressure ammo. I have seen several modifications to the cylinder to fire .32 ACP, .32 H&H Mag, ect.. Due to the Gas Seal cylinder design and the fact that you will find Nagant with barrel dims from .310 to .313, I personally don't feel comfortable dropping the hammer on one of these veterans.
My .02 cents