Magic Rockwell number is 45. actually, between 42 and 50 should be fine.
I think it's your tunion. Can you please post some close-up pics of it? especially the parts which the bolt locks against? I think they're out of spec, thus basically hammer-forging your bolt into a different shape every time you fire it. In any case, this can be very dangerous as once those nubs lose enough shape one of two things will happen-
A: too much of the back of the round will be exposed, meaning it will be like shooting with too short of a headspacing. That will probably mean 'boom' instead of 'bang'..
B: the bolt will slip through the back of the trunion, turning your AK into a blow-back weapon. Last I heard, the blow-back weapons' recoil is alot more noticable than with a locked bolt.
In either case, it can be dangerous (esp given the fact that even if the bolt carrier doesn't fly out and hit your heat, that dust cover is thin, soft, held by almost nothing in the front, and more importaintly hindged on the back).. The former is really what you should worry about because that would normally happen long before the latter.
In any case, I'd get that checked out ASAP. so please post pics of the trunion and trunion with bolt- preferably with you pulling the bolt back (to show space), and even better with a round or shell casing inserted just so I can see if you've got that unsupported effect going already.