Under pressure and Army boots . . .
5.56 NATO vs .223 SAAMI “…the throat really matters. NATO is loaded to higher pressures than SAAMI. This can be to the tune of 15,000 psi difference. That much, folks, can cause ruptures and even upper receiver cracks. Another issue is NATO rounds can sometimes stick the bullet into the lands in a SAAMI chamber. That radically raises pressures.” – …