It will take a 308 no problem the heavier grain is hard on the gas system stay 168 and less or turn the gas chamber off and hand rack it if you use 180. the pressure is the issue not bullet weight or size nato vs 308
i don't know about you, but all of mine have been an absolute b*tch to open without the gas assist- i've done it a few times just to see what it's like- that list includes real m14s as well as norcs- there's just not enough steel on that tab to get a good grip and pull to the rear at the same time and i waited a few minutes between rounds to let the pressure in the chamber diminish- just stick to the 165-168s top end and you'll be fine- btw, with handloading, i get groups of 1 inch and under at 100 yards- about the size of a loonie for a 20 round mag