CapnMac
Member
The distinction between .223 and 5.56, is, functionally, similar to that between .308 and 7.62x51 (e.g. 7.62nato).This is my first rifle that's fired both. So I'm a bit green behind the ears if you know what I mean.
Which does not prevent massive flame wars online and all manner of dire warnings in every possible direction.
In the present case, generally, a 5.56 labeled barrel will handle .223 ammo.
Where the distinction lies, as noted well above is in the twist rate of the barrel used.
Which unlike most other arms in a given caliber has seen significant changes over the fifty year history of the 5.56nato/.223--and not in a uniform way, either.
Note the Venn Diagram above. There are reasons that people will still want to use 1:12 twist barrels. So, it's not like the slower twist barrels have completely replaced fast twist. Well, other than the serious long range shooters--where ling heavy bullets are king.
The twist rate on your barrel is what will matter most. It should be stamped in to the metal some place visible--either at the breech end or muzzle.
And, as noted above, the best this is to get a hold of several brands, and bullet weights, and see what you shoot in your rifle best.