OP, I've done this for years without any issues and so have hundreds of other people. There is enough powder in those surplus 7.62x39 rounds to fill the case to a non dangerous level IMHO.
If your really concerned with SEE, go to your local fabric shop and get a couple of square meters of Dacron or similar fiber. Cut it into 1cm squares, which is easy because it's appx 1cm thick.
After pouring in the surplus powder, insert a square of fiber, to hold the powder against the primer. This serves two purposes and is a practice I follow myself.
I haven't been worried about SEE, but as mentioned, inconsistent burn rates can and do create accuracy issues. By holding the powder back against the primer port, you get a consistent burn/initial ignition. It also alleviates any chance of SEE.
You need to be careful, not to use a square of fiber that is to large. It can cause a bit of fouling.
One other issue with the surplus powder in the x39 is that it isn't very consistent from lot to lot, sometimes even in the same lot.
Both Chinese and Soviet x39 surplus is loaded with a spherical or "ball" type powder that is almost identical to AA1680. It's often found in bulk, labeled WC680.
It's a pretty versatile powder and if you have enough, it can be used for several other cartridges as well.
Higginson's used to sell it for under ten dollars per pound and about half that, when they cleared out their remaining stock.
Because this powder isn't commercial "cannister" grade powder, it often varies slightly in burn rates.
Such rounds make for very laid back practice rounds. Don't expect stellar accuracy out of any of them, especially if you're going to utilize the surplus bullets as well. They are slightly undersize and the FREEBORE is far to long at best
