BudsGunShop.com - .223/5.56

The two cartridge cases are identical but the way the chambers are cut (the leade) and the way the pressures are measured are different. The .223 is safe in the 5.56 chamber but the 5.56 is not safe in the .223 chamber. Your upper will shoot either.

Yes this upper is interchangeable with both 223 and 5.56

Great question remember 5.56 is a more powerful​ ammo. If the upper does not say 5.56 I would not recommend you using 5.56 it's best if you just use .223 unless other wise stated. But this upper seems to be only .223

CMMG's website says it's good for 5.56, and if it's okay with that, .223 is perfectly safe.