I'd like to build on what Old Fred has stated.
According to my Cartridges of the World, SAAMI did not recognize the difference between 223 and 5.56 until (okay, now my copy is hiding from me...but sometime it the late 70s, and as the 223/5.56 was invented in 1964...that's more than a 10 year gap when they WERE the same)
so for quite some time 223 WAS the same as 5.56, they were two different names for the same thing, just like 45 auto and 45 acp.
Now, eventually the army switched from M193 to M855, with a heavier bullet and the military started asking for slightly different chambers.
So I think it is more along the lines of .223 remington is the same as OLD 5.56 and different from NEW 5.56.
I will also note that SAAMI frequently pussyfoots around and plays it extra extra safe in pressures. I think they have a vested interest in asking for older catridges to be loaded softer so that whatever new whizbang they are each individually making looks better compared to the old standard.
Remember, SAAMI is NOT government, it is simply a group of ammo makers who agree to follow a certain set of self-chosen guidelines. They look out for their own best interests. This includes making the new wonder 7mm short case look better than the tried and true 7x57mauser, etc.
Also remember, the US military uses 9x19mm pistol ammo that is above SAAMI pressure guidelines. I don't think anyone is going to try and say 9x19 'civilian' is different than 9x19 'US Military' even though other countries do load special 9x19mm ammo with even higher pressure yet for SMGs.
EVERY gun that rolls off the assembly line marked 223 is going to be able to handle European Loaded 223, and again as Old Fred stated, the European SAAMI views these rounds as the same, and allows 223 to be loaded a tad over 62,000 PSI.
I will point out for handloaders it is important to realize military 5.56 brass is much thicker, so you actually have less case capacity. If you don't realize this you could accidentally overload the case.
