AR15 5.56 NATO on the barrel and 223 receiver

The barrel/chamber is what matters. It doesn't matter what the receiver is marked, if at all. If the barrel is marked 5.56, shoot both 5.56 and .223 to your heart's content.

For what it is worth, I'd happily shoot 5.56 in a .223 marked barrel anyway. Odds are you'll be shooting .223 because that's what is available.
 
Military chambers are usually 'looser' in dimensions so 'bent' ammunition will still chamber. I doubt you would ever have a problem with commercial ammo.
 
.223/5.56 are functionally identical in every way and completely interchangeable. Don't beleive the BS people post about them being magically different. Same with .308 & 7.62. Same round, different name.
 
.223/5.56 are functionally identical in every way and completely interchangeable. Don't beleive the BS people post about them being magically different. Same with .308 & 7.62. Same round, different name.

Except the military chambers have larger tolerances and the military ammo is sometimes crooked as hell. I have chambered 308's to minimum saami specs and some military ammo would not come close to chambering... and yet those rounds would chamber in a sloppy chamber. Roll a round on a smooth surface and it would have a big wobble in the body of the case.
 
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