It gets back to the bullets. It doesn't sound like you intend to add the weight of a suppressor, so the subsonics are probably of no interest. The problem with shooting the more standard 123's with the fast twist are exaggerated wobble in flight (eccentric spin due to small mass asymmetries in the bullet) and the potential for core stripping (when the bullet jacket loses its grip on the lead core due to too rapid angular (rotational) acceleration, and that leads to groups opening up. This can happen to .30 cal in a 10" twist gun at velocities of around 3100 fps, and with cheap bullets whose cores are already loosened by a deep collet-type factory crimp, it may be as low as 2700 fps. This is one reason cheap ball ammo groups are sometimes 3 or 4 inches at 100 yards, when the Hornady FMJ's you carefully handle and load yourself can group 1". It's tempting to warn you 8" twist could do that at 20% lower velocities, but the 7.62×39 has about 20% lower peak pressure and therefor lower peak angular acceleration, so you still may not see it at all. I'm just bring up the theoretical possibility.