A rifle with a .223 chamber isn't built to handle 5.56 pressures. Regular use of 5.56 ammo can damage your rifle.
If it were marked "5.56 NATO" you'd be hunky dory to shoot either or through it.
Sgt. of Marines, 5th Award Expert Rifle, 237/250
Expert Pistol, 382/400. D Co, 4th CEB, Engineers UP!!
If you start a thread, be active in it. Don't leave us hanging.
OEF 2011 Sangin, Afg. Molon Labe