SAMMI started this, and I'll explain.

223 Remis a SAMMI controled round, 5.56 is not.

5.56 is loaded to 60,000 Cup and SAMMI has 223 Rem restrictedat50,000 Cup, thus your higher preasure answer.

5.56 has a max case length of 1.77 and 223 Rem 1.76.

Bang away with military ammo all you want, any reloader will tell you to get the best accuracy out of a gun the bullet must be set further out than SAMMI standards recomend's andcloser to the lands and groves, and if it fits in the magizine it will work most of the time. Furthermore if the bullet is too long the bolt with not close right.