You can fire 223 in 5.56 with no problems. The cases are exactly the same. The chamber throat is the only difference. The pressure of the 223 seems higher but isn't because it is taken at a different place in the barrel and on a different scale for NATO specs. Velocities are the same so pressure is the same.

You are correct Joseph. One should always follow manufacturer suggestions for warranty/legal issues to be covered with no question. You can find experiments with tens of thousands of fired 5.56 rounds fired through 223 barrels to prove the practical point that there were no ill effects noted. But, as a rule of thumb, always follow what the manufacturer recommends. Please look up 223/5.56 and Wylde chambers and you will understand that it's a matter of case "seating" that separates the 3. 5.56 barrels are no harder than 223 barrels. The real issue is the distance the bullet has to "jump" before engaging the rifling that could potentially cause a pressure spike in the chamber. Always better safe than sorry.

And a wise opinion too. Many people fire +p in firearms they shouldn't. Its the manufacturer being cautious and the consumer finding ways to press our luck.