Did some math to compare bolt thrust between 5.56 and 7.62x39
5.56 NATO,
Case head diameter .378"
Case head area .112 sq-in
Max pressure 62K psi
bolt thrust: 6,944 lbs
7.62x39,
Case head diameter .447"
Case head area .157 sq-in
Max pressure 45K psi
bolt thrust: 7065 lbs
Measured the rear lug surface of both bolts. They are the same.
This is all valid. Now consider the difference between force on the lugs in a barrel with all bolt lugs seated evenly, and one where the barrel is not square to the bolt and one or two lugs take all the pressure.
That, combined with less support of the lug bases on the 7.62x39 bolts, is what really breaks bolt lugs. Squaring up the receiver (and therefore the barrel extension) goes a long way to making the bolt live. Getting the rifle properly gassed to prevent premature unlocking during high pressure (as you were saying) is a good thing as well. If you do those two things and use a good bolt, you can expect good bolt life.
