[gusty]

An AR-15 can be either 5.56 or .223. A 5.56 can use both 5.56 and .223, while a .223 AR-15 can use only .223. Something to due with the tolerances of the rifle.

[castlebrew]

“An AR-15 can be either 5.56 or .223.”

...and neither can use .233...

[Spktyr]

Not true.

http://en.wikipedia.org/wiki/5.56x45mm_NATO#5.56_mm_NATO_versus_.223_Remington

You have to check to see which chamber type it has as many weapons marked .223 Remington can actually use 5.56 safely.