Originally Posted by sandog
Not just hotter but 5.56mm also requires a longer throat (freebore) to keep pressures from being excessive. .223 chambers have a shorter throat and steeper onset of the rifling to help accuracy, 5.56mm has a longer throat (area before the rifling starts) and a more gradual onset of rifling to keep pressures down with longer bullets such as the tracer round, reliability being above tack driving accuracy for the military usage..
Great explanation thanks! Tagging this for future reference.