I know that there is a difference between these two rounds.  But what is it.  I think you can shoot .223 on a upper stamped 5.56mm, but you can not shoot 5.56mm in a upper stamped .223.
Also do the lowers differ between the two calibers.
I an going to build my own rifle and i dont want it to blow up on me.
BTW the reason im asking this question in this board insteed of the General one.  I trust you all.  I know that the guys in here know what there are doing when it comes to AR's  

Thanks
Mike

Also, lowers that work with .223 will work with the 5.56.
And visa versa.

Thanks again to all that have replyed.  That was a quick responce, I really appricate it.  

What are Cav Arms chambered in?

Mike

The SAAMI specs for max pressure is higher in the 5.56 than in the .223. The chamber in the 5.56 is cut with a longer throat to help with the added pressure. 5.56 brass is also heavier than .223. 5.56 rounds shot in a .223 chamber will show pressure signs on the brass and increased throat erossion in the barrel.

So with all this being said.  As I looked through the M 1 sales catalog that I picked up yesterday.  I wondered why do they even offer there uppers in .223 along with 5.56?  I mean why would I want a substandard chambering? I.E the .223.