Sorry, this has probably been covered several thousand times.  Whats the difference between the two calibers.  I always thought they were the same, only one was the Nato version.  I have my lower built and am looking for an upper.  

Is there a difference to worry about?

Is one more interchangeable, like shooting .38 spcl out of a .357, but not the other way?

Or is it nothing to worry about?

This place will help a lot!

link

http://i2.tinypic.com/ouy73n.jpg

For the most part, .223 can be fired in a 5.56 chamber, but not the other way around.  Your Mileage may vary though.  Generally, the 5.56 NATO spec chamber is a little looser and specefies higher pressures.  Shooters going for gnat's ass accuracy would want to shoot .223 ammo in a gun chambered in .223.  If, however, you're more concerned with a reliable rifle that will happily digest whatever Mil-surp or commercial ammo you throw at it, you want a gun in 5.56, and shoot whatever ammo you can get inexpensively.

The cartridge cases themselves are effectively identical with the exception that military 5.56x45 NATO specification rounds may have thicker cases and are thus able to withstand stretching better when fired in loose chambered military weapons... From the data I have seen the headspace specifications are identical for SAAMI .223 and military 5.56 NATO ammunition although measured to different datums and the dimensions of the cartridge cases and chambers are for all intents and purposed identical, they just have minor tolerance differences...

Commercial SAAMI specification .223 chambers have a much shorter throat, a smaller diameter bullet seat, and less freebore than the military 5.56 NATO chamber... When military specification ammunition is fired in a SAAMI specification chamber, chamber pressure can increase dramatically, sometimes up to 10,000 psi... While this should not cause any problems in modern rifles it should give one reason to not take any chances... With SAAMI specification commercial ammunition, when fired in a military chamber will have significantly lower pressures and velocities...

Quoted: This place will help a lot! link i2.tinypic.com/ouy73n.jpg

Holy #%@!  Great info! I love this site.

That is a great link!  I would like to thank everyone here, from the step by step build pictures, to chambering choices.

Thanks