A milspec trigger with do fine with surplus steel cases if you just remove the firing pin from your bcg, and use your reloading or other calipers- carefully measure the thickness of the shoulder where it stops in your bolt to allow the actual striking face of the pin to hit the primer. Put it in a drill press and "polish" off or turn the leading edge of the shoulder of the firing pin .01 to .017. I use some valve grinding compound, emery cloth, and lightly polish after to a mirror checking the measurement as material is removed.
I've built a half dozen x39's and that has always made a world of difference. You can experience going deeper, thus allowing more protrusion of the firing pin and a harder, deeper hit, but too much can overpenetrate and puncture a primer. I've set most at .015, and had success on all ammo, including the red lacquered over yellowish surplus stuff from the early 50's
Now,
if you Really wanna shoot 7.62x39, there is this one other platform that does it Very reliability...