I've already addressed this in a couple other posts recently, all based on numerous reports we got when I was with American Rifleman magazine.
The only time a slam fire becomes truly likely (barring the bolt being incorrectly assembled or a really filthy rifle) is when the bolt and particularly the firing pin have been lubricated with one of the new super type lubes.
I'm trying to find out whether any of these lubes have caused slamfires with military surplus ammo or not. Normally it's with current production ammo, though.
I've put approximately 7,500 rounds, both surplus and commercial, through my SKS since I got it in 1992, and have yet to have a slam fire.
But, I shoot with the firing pin dry, and only use natural oil on the bolt, nothing with teflon, moly, or anything like that.