ScottA is right about the pressure but there is a little more to it. The 5.56x45 chamber has a longer leade (distance of the bullet to the riffling) than the 223 to compensate for the higher pressure which is only about 10%. The bad part come when you load the higher pressure round in the shorter leade of the 223 chamber. A 10% over pressure can jump to 30+%.
"I would not be an old man if I had not been an armed young man." JTJ
Patron Member NRA
"If you have always believed that everyone should play by the same rules and be judged by the same standards, that would have gotten you labeled as a radical 60 years ago, a liberal 30 years ago and a racist today"