Mexican Match ... do it all the time, but you have to be smart about it. First, if you haven't done this, look twice, cut once.
First,
Remove the operating rod in your SKS.
Next,
Pull out 40 123gn bullets. Now, dump the powder for 10 of them and get an avg charge weight. Now dump out ALL of the powder from all 40
This is the serious part!!!!
Since you do not know what the powder is or its characteristics, charge the first 3 cases with a charge weight a FULL 5 GNS below the avg charge weight you calculated when you weighed the first 10.
Charge the rest of the cases in batches of 3, increasing in .5gn increments. You should not get to the original charge. YOU MUST STOP AT LEAST 1 GN SHORT OF THE ORIGINAL CHARGE WEIGHT (AND EVEN THEN i DON'T GO THAT FAR)!!!
Now, go shoot your work-up loads watching constantly for sighns of pressure. Since you have removed your operatinig rod, the rifle will act like a bolt action, you will feel a hard extraction if you get it. Look at the primers, case bulging, bolt marks, etc...ALL OF THE NORMAL PRESSURE INDICATIONS.
This is not a recommendation ... Having done this hundreds of times now, I settle somewhere in the vicinity of removing less than 2 gns of powder to give me what I want. That doesn't mean that that will automatically work for you.