As I understand, while both .223 Rem and 5.56NATO rounds will chamber in .223/5.56 rifles, but rifles designed for 5.56NATO have longer leades to accommodate the slightly higher pressure of the NATO round. I have a bolt rifle in .223 Rem and was warned to not use 5.56 NATO.

from:5.56 and .223: Are They Different?
Quote:
...The leade on a proper 5.56 chamber is twice or more than that of a .223 chamber, and the onset angle of the 5.56 rifling creates a ramp with four times the distance. Firing .223 in a .223 chamber, or a 5.56 chamber, is not a problem. But firing real-deal 5.56 ammo in a .223 chamber can be a big problem.

The SAAMI-spec maximum average pressure for the .223, measured at the middle of the case, is 55,000 psi. The NATO spec for 5.56 is determined by SAAMI’s European counterpart, CIP. CIP measures at the case mouth and lists the 5.56 pressure spec of 62,000 psi. Measured at the case middle as SAAMI does, it shows 60,000 psi—so either way it’s higher than the .223. ...


I'm sensing thread drift....