However, with equal weight bullets, the 7.62x39 will have a significantly lower velocity.
Absolutely correct. The Short Russioan carries a 125-130 gr bullet at something like 2300-2500 fps. The .30-30 carries a 150 gr bullet at about 2200-2300 fps.
If the Russian were loaded with 150 grain bullets, there would b less room in the case and thus less powder. Lower velocities would be the result.
Again, it wasnt intended to be a powerhouse. Merely functional out to 100 meters, + or -