I do not know for sure but I think it has more to do with the chamber of the rifle then the bullet it self . There still has to be something else though .
Just copied this from another site .I found it on the internet so it must be true
There is some overlap in the rounds, Im not sure of excact numbers but basically the 5.56 maximum overall length is longer then the max overall length for .223. So if you have a rifle chambered on the short side of .223 specs some 5.56 ammo wont work or be way over pressured when fired in the shorter chamber. If you are buying an Autoloader for volume shooting you should get a 5.56 chamber so you wont have problems with military surplus ammo
There are some 223 Remington rifles in which you wouldn't want to shoot 5.56x45 cartridges, some in which you wouldn't think twice, and some that you'd want to check before trying.
CIP standards (Europe) require pressure testing be done with the transducer at the case neck, where SAAMI cartridges typically measure with a transducer at the case head. You really can't translate directly from one to the other, but it's possible, perhaps even likely, that NATO loads will be at a higher pressure than is permissible in 223 cartridges, though not by a huge margin.
Also, it's been a long time since the standard military loads included a 55 grain bullet, and leades in military barrels tend to be pretty generous, giving up a little potential accuracy with those bullets to allow room to chamber the longer, heavier bullets common in military loads. Some civilian 223 rifles may not have the freebore to allow those bullets to be chambered safely, and if you're jamming the meplat of the bullet solidly into the lands, you're going to raise pressures.
If you have a strong bolt action like the Remington 700 action, it'll take a good deal of overpressure, though it isn't smart to get too smug about it. Additionally, if you want to be obsessive, you can measure the leade in your rifle to make sure there's a little room for the bullet to jump before it hits the lands. There are several ways to measure, some rather tedious, and probably the simplest/safest using a little gizmo made by Stoney Point.