Crazy, I've got 3 'rincos that have eaten thousands of rounds with no problems. I thought they tended to smack the primer too hard, if anything. Hope you find a fix, not just a swap. I hate a mystery...
Weak hammer spring would be my guess if it works fine with commercial ammo but not milsurp. Certainly NOT a common problem with SKS's, but not unhear of either. Pretty common with commercial rifles however.
gunsprings.com
Finally I'm not the only one with this SAME problem. Mine is a chinese SKS-D. I just got my russian one from SIR this week. I was thinking that if it shoots well I might swap out the bolt and firing pins to see if that is the problem. I'm not sure if that is possible??
I am working at the moment and reading the fourms on my blackberry (not the most user friendly way to do it). I will search for my thread I started and maybe something in there will help you. To this day I have not fixed the problem and it driving me NUTS.
is it a big job to replace the hammer spring?