I am wondering what the recommendation is on 5.56mm ammunition and .223 ammo.  At first I was under the impression that they were the same thing, just different systems of measurement.  I am building my own lower, and am starting to look for an upper; this is my first AR so I am still wading through all the information.  My main questions are as follows:
1. Is there a difference in the two rounds listed?
2. If not are there some barrels/uppers that shoot both .223 and 5.56?
3. If not what are your recommendations for choosing between the two?

Thanks a lot in advance!

www.ammo-oracle.com/

You want 5.56mm.

Quoted: You want 5.56mm.

. . . unless your building a match rifle.  In that case you would want a Wilde or .223 chambering on your barrel.

5.56 is the best bet;  It will easily swallow all 5.56 and .223 ammo.

M.L.