Originally Posted by Travclem
5.56 NATO is around 60,000 psi and SAAMI .223rem is 55,000psi.
Pressure is important but so is rearward bolt thrust. You need more diameter (larger case head)/larger bolt lugs and bolt mass to control all that. Bigger round = bigger action because the build materials are the same. Compare the Thompson Contender and Encore frames for example. One can handle .308 and the other can not.