Headspace and op rod timing matter!
1. Chamber pressure differences between 308 and 7.62 is nonexistent in terms of safety for the design of this weapon; bullets heavier than 175 grain can have to much port pressure for the M1A/M14 platform; NM shooters solved this by putting a groove in the gas piston to relieve it. Chamber pressure is different than port pressure, high port pressure leads to battered receivers, cracks in the receiver heel, bent op rod, and this is the big one for this discussion, case head separation. The extraction process becomes more violent because of higher port pressure.
2. Headspace is what rules here. Brass in 308 is thinner resulting in about a 10% increase in volume, this thinner case wall is easier to pull the head off. Case separation is due to chamber size compared to case size, how far the case stretches, the violence of the action during cycling, surface condition of the chamber, pressure curve in the combustion process, op rod timing, and gas cylinder port pressure.
3. The M14 has a violent action during extraction; its design included a chrome chamber to assist in extraction, ease of chambering a round, dirt resistance, easier cleaning and errorsion suppression.
4. The headspace for the 308= 1.630-1.634
The headspace for the 7.62= 1.635-1.640
If you shoot a 308 round in a 7.62 chamber, if the headspace is long in that 7.62 chamber, say 1.339 and the brass used in the 308 is thin and hard it can lead to case head separation during action extraction cycle, reloaded 308 brass resized can make this problem express itself more often.
You can shoot 308 in a 7.62 chamber, you can shoot 7.62 in a 308 chamber, I would not shoot 308 ammo using heavy bullets, thin hard brass, in a non chrome chamber because the risk level has been raised for a case head separation.
I would not shoot a surplus 7.62 in a tight chambered SAI action, read 1.362 or less, non chrome lined chamber, the possibility increases that an out of battery firing can occur. Chrome not only aids in extraction it also aids in chambering a round.
I have said and will say, 1.634-1.635 is the ideal chamber size to fire both, it does not prevent case head separation but it lowers the risk, it does not prevent out of battery firing but it lowers the risk.
The safety factor designed into the M14/M1A is remarkable.
For all of the failures, I have never heard nor seen a serious injury. I consider serious as loss of life, loss of eye sight, missing fingers, hands, or other body parts.
This debate revolves around part fact and part myth. There is a difference between 308 and 7.62x51, it lies in the case wall thickness, case head design, case hardness, and chamber dimension. The safety features inherent in the receiver design protects us fools who push the limits of this incredible design.
Jim