Hi everyone, y'all have been loads of help for newbies like me.  I've been reading the forums, TM's and studying pictures for weeks and finally started getting parts for my first build.  I thought I had things squared away so far but suddenly got confused with the whole .223 vs 5.56 issue.  I have so far a Colt surplus barrel ass'y (NOS, 20", thin, triangle guards), an A1 upper receiver (no internals yet), and ordered the RRA U.S. marked lower from LT.  
  My question is this; I don't see any markings on the barrel or upper as to their caliber - what can I check/measure to verify what it is?  Also, what do I need to keep in mind for my build if I want to gear it toward the 5.56? and do i really need to concern myself with the .223/5.56 difference for just a shadetree shooter?
  I will have to install the barrel onto the receiver and plan on getting a new bolt/carrier assembly so what are the chances of the headspace being wrong? and does that relate to the .223/5.56 issue at all?

Thanks a bunch for any thoughts and suggestions!
Curt
 

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The Ammo Oracle has the answer!

The short version is no, it shouldn't matter to you.  5.56 chambers are a little looser than .223 chambers, but as a practical matter it's almost the same thing.  Either way, it's not a headspace issue.