Now I have been under the impression that these two rounds are the exact same thing,  .223 referring to the American measurement system (and thus domestic hunting and sportsmanship) and 5.56 is the size in Metric used internationally and by our military. Now someone told me you can get ARs in .223 and 5.56, and they actually have different barrels.... what? Can someone clarify this, are they the same or are they different... also what about .308 and 7.62x51 now?

,308 and 7.62x51mm are essentially identical and can be interchanged.

Quoted: Now I have been under the impression that these two rounds are the exact same thing,  .223 referring to the American measurement system (and thus domestic hunting and sportsmanship) and 5.56 is the size in Metric used internationally and by our military. Now someone told me you can get ARs in .223 and 5.56, and they actually have different barrels.... what? Can someone clarify this, are they the same or are they different... also what about .308 and 7.62x51 now?

The rounds are physically the same, but the .223 Remington chamber and the 5.56 NATO chamber are not.  From the Ammo Oracle;



www.ammo-oracle.com/body.htm#diff

This link explains the differences between SAAMI chambers (.223) and 5.56 NATO chambers.

So they are not interchangible? So which is more prefered, avalibility of ammo wise?

Just read it now

So bassicly a 5.56 rifle will be able to fire both Mil-spec (ie surplus) and .223 (like remington) while a rifle made for .223 will not be able to shoot 5.56 (not enough room). Right? So go for a 5.56mm then?

Quoted: So they are not interchangible? So which is more prefered, avalibility of ammo wise? Just read it now So bassicly a 5.56 rifle will be able to fire both Mil-spec (ie surplus) and .223 (like remington) while a rifle made for .223 will not be able to shoot 5.56 (not enough room). Right? So go for a 5.56mm then?

Ya

The 5.56 NATO is a particular loading (or rather, several loads) of the .223 Remington.  The 5.56 is loaded to higher pressures than the SAAMI spec.  

The SAAMI spec was established using the "weak link" of finding a load that would be safe in the weakest production rifle at that time.  

However, the AR-15/M16 is quite stout, and is made for firing the higher pressure 5.56 NATO spec rounds.  Gazillions of 5.56 rounds have been safely fired through .223 chambered AR-15's/M16's.

Opinions vary among manufacturers on this issue.

Truthfully, this just has not been a problem, otherwise, there would be a lot of injured people and lawsuits.

I have fired thousands of 5.56x45 in several 223 Remington Rifles never had a problem, not saying you couldnt have one just never have.