>> I still wouldnt shoot a 5.56 out of .223 gun. No, the rounds are different and I don't mean to imply that they are identical. Only that the spec of the difference in pressure is overblown. (Ahh, see what I did there?? "Overblown"!) Something like the .223 Wylde chamber is specifically made to accept either round. Rounds can be different by manufacturer and can be different by product line. But on an apple to apple basis I'd compare two similar rounds. Like the Federal [American Eagle Rifle 223 Rem](https://www.federalpremium.com/rifle/american-eagle/american-eagle-rifle/11-AE223JX.html) with 55 grain bullet and the Federal [American Eagle Rifle 5.56x45mm](https://www.federalpremium.com/rifle/american-eagle/american-eagle-rifle/11-XM193X.html) with 55 grain bullet. The .223 is listed as Muzzle Velocity of 3240fps and the 5.56 is listed as 3165fps. In that round with the same bullet mass, the .223 has a higher muzzle velocity. Now, I won't try to tell you what I don't know. So below is my understandings and/or speculations; **I understand** that SAAMI is more rigorous on testing and insisting on compliance with published standards whereas making a NATO round is more free-for-all. That leads to some makers (or lines within a brand) to 'overcook' their 5.56 rounds to deliberately make them shoot hotter. That means that some lines offer the equivalent of +P rounds in the 5.56 products that are beyond the published pressure spec. There is also a difference in the way the primer attaches to the case in .223 vs 5.56 rounds. It could be that one is more rigorous than the other and that 5.56 is less likely to fail with these '+P' pressures.