I'm looking at buying a complete upper to go with my RR lower.  I noticed that complete uppers with chrome lined barrels and chambers are listed as 5.56 NATO.  Haven't seen one stating for .223.  I always thought 5.56 and .223 were interchangeable.  ???????????? If I get one of those uppers will .223 operate in it OK?

Like I said, dumb question.

Quoted: I'm looking at buying a complete upper to go with my RR lower.  I noticed that complete uppers with chrome lined barrels and chambers are listed as 5.56 NATO.  Haven't seen one stating for .223.  I always thought 5.56 and .223 were interchangeable.  ???????????? If I get one of those uppers will .223 operate in it OK? Like I said, dumb question.

Yes, .223 will operate fine in a 5.56 NATO chamber.

.223 A.O.K. in 5.56...the reverse not necessarily so, however.





Only time you have to be concerned is using MILSURP ammo in a "match"-quality .223 chamber.

See Ammo Oracle
The .223 chamber is often used on match grade barrels, and since chrome lined bores do not usually give the best accuracy you would not normally mix a .223 chamber with a chromed bore.
While a chromed bore will give you better reliability in bad and dirty conditions and more than enough accuracy for the average GI Joe, if you want 1/2 MOA or better groups at 600 yards GENERALLY one would go with the unchromed bore and .223 chamber.

Holy cow.  Never realized there was those differences mentioned in the Ammo Oracle.  Glad I asked that dumb question.  Thanks all.