Well, according to the load data I found just with a quick look, it seems 27 grains of H4895 should be just fine with 123 grain bullets. The 7.62x39mm case is pretty small, so it's very likely that you will simply have a compressed load. The 123 gr bullet shouldn't encroach that much past the neck of the case, and the neck of the Russian is pretty short, so I doubt you'll have too much compression. On Hodgdon's load data site they actually show 27 grains of H4895 as the starting load for 150 grain bullets, with 28 grains being the starting load for 123 grain, so I'm guessing you have nothing to worry about.
One thing to look for in compressed loads is bullet creep. Basically, if you compress the powder enough the pressure can, over time, push the bullet out of the case a bit. I'd load them and then measure COAL. Then before you fire them, I'd measure it again to see if it's grown. If so, you need to reseat the bullets and crimp, or use a faster powder that takes up less space in the case.