You got me curious. I went down to the basement and dug up a few old reloading books. Most did not mention the 5.56, the Sierra and Hornady books both did, they both state that the 223 Remington began as a military cartridge in 1957.
Sierra says nothing else about it other than a note on the bottom of the page to reduce charges by 1 grain when using military brass. Hornady lists no change in charge. Neither manual shows a difference is case length ect.
The difference in not in the case, rather in the chamber of the firearm itself. The 5.56 has a longer leade that allows for a longer ?ogive? on the bullet, which is loaded by many countries in their military ammo. Using this type of bullet, or longer OAL on the cartridge by having more bullet above the case mouth, can jam the bullet into the rifling which can result in pressure increases of as much as 20,000 PSI. This is a proof load for the .223 and too much of it can be a problem. The biggest issue is that if you ignore the early warnings (flattened primers, gas leaks, difficult extraction, etc.) you may experience a catastrophic failure when you finally overstress the firearm.