[P-990]

5.56 has a slight larger chamber throat and longer leade. This gives a little more tolerance for mil-spec ammo at the case necks and bullet ogive.

As far as brass capacity, most Lake City 5.56 brass actually has more internal capacity than civilian .223 brass. We Highpower shooters tend to value it because of this attribute (and it's incredibly tough stuff). And you can use Lake City cases in a .223 chamber by simply resizing in a .223 die and proceeding as normal.