The 5.56x45 NATO (North Atlantic Treaty Organization) caleber is very close to the .223 yet very diferent. In a standard ar-15 rifle, 5.56 NATO or .223 can be effectively used. However in most sporting rifles only the .223 is recomended for use. The main reason for this is the load. The 5.56 NATO round provides more power that it is too powerful for the .223 chambered gun to withstand for any type of prolonged use. Therefor 5.56 is better for use in any gas opperated/blowback action gun. Also the 5.56 round has a thicker caseing (because of the 'hotter' load) which is better for reloading (because of resistance to cracking).
The 5.56x45 NATO also has one other major use. The 5.56 NATO spec round is the cartrage used by not only the US forces but those encompassed in the NATO aliance. The ammo usually used as NATO is manufactured by Lake City Amunition in Independence, Missouri. Along with this amunition the army also uses the 7.62x51 (.308) which is normally refered to as a sniper round. The .308 is also somewhat close in specs to the .257 roberts used alot for 1000 yard competion.
The ever popular .223 has recently been challenged agenst the 6.8spc or 6.8x43 which is derived from the 30 win. cartrage. The 6.8spc has an advertised speed of 2800 fps but pretty much is maxed out to 2650 from a standard ar-15 with a 16" barrel which is slightly greater than a standard .223 but when loaded into a magazine, takes up more space thus limiting the mag capacity to less than what a .223 mag can hold. If you have a .223 mag holding 30 rounds you can expect to fit about 25 rounds of 6.8spc. The takedown power of the 6.8 is simaler to that of the .223 being able to take down a human foe at 300 yards however the .223 with its popularity, ease of production, and power simply will never be replaced by the 6.8spc in my opinion.