this 1 is a bit more indepth with actual testing..
http://www.luckygunner.com/labs/5-56-vs-223/

some of the highlights

However, I was surprised about the results I saw. There was no dangerous spike in pressure from firing 5.56mm ammo in a .223 chamber. While pressures were elevated in comparison to one of the 5.56mm barrels, they were slightly below the other 5.56mm barrel.
It’s important to note that most of the .223 chambers I’ve fired 5.56 in didn’t show any signs of excess pressure
Summary: The majority of the experts I consulted over the course of my research did not feel that there was a major difference between .223 and 5.56 chambers in terms of pressure.