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This is from a contact at ATK / Federal Cartridge. Italics are mine:
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5.56 vs/ .223
To ensure operational reliability, the 5.56 round is designed to operate at significantly higher pressures, namely just short of 59,000 psi (case mouth meaured) which equates to almost 62,000psi when measured via conformal transducer (as done by SAAMI).
The SAAMI max set forth for the 223 is 55,000psi (conformal transducer).
To facilitate operating under these higher pressures, a 5.56 cartridge case possesses a stronger web design (thicker case walls towards the head), reducing internal powder volume, an important consideration for reloaders.
The remaining differences are found in the chambering of the weapon. A 5.56 chambered weapon differs from a .223 in the throat and freebore. A 5.56 throat is approx. 30 thousandths of an inch longer and 3 thousandths of an inch larger in diameter than a 223 chamber. In addition, the throat or start point of the rifling, the taper with which the lands begin, is at a lesser angle, more gradual in a 5.56 than in the .223.
These rather small, but very important differences, allow peak pressures to drop adequately before the bullet engages the rifling and also allow for more build-up of fouling and shooting residue without impeding function.
What does all this mean? In short, you can safely fire all 5.56 AND .223 ammunition in a gun properly chambered for 5.56. You MUST NOT fire 5.56 ammunition in a 223 rifle. As case in point, I fired XM193 5.56 ammunition in a .223 test barrel with average pressures (conformal transducer) of 72,550 psi, and peak pressure registered at 76,250 psi. Continued shooting of 5.56 ammunition in guns not chambered for 5.56 will show many warning signs of over-pressure, such as flattening of primers, smearing of the head stamp, dropped primers, blown primers and pre-mature wear on extractors and bolts.
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Well done
