Thought it was known that military brass was thicker than commercial, making interior volume smaller. Raising pressure.
Isn’t that the case?
The 556 is loaded to higher chamber pressure than the 223. When fired in a 223 chamber (shorter leade) the gun experiences higher pressures than experienced in a 556 chamber.
Physically, you would have difficulty measuring the physical difference on the 2 rounds (the specs overlap), the difference is in their interior ballistics behavior.
Best accuracy is seldom at max loading for most ammunition.