Honestly if it doesn’t get a bigger mag having 800 ROF is just shooting yourself in the foot.
I think that’s a realistic way to balance it and make the AVT-40 better than the AVS-36 again. The AVT-40 was able to use 20 round mags irl, and it has a slightly lower fire rate, so giving both weapons their irl fire rates and giving the AVT 20 round mags would make the AVT-40 significantly better than the AVS-36 and make them both perform realistically.