Originally Posted by flw
Rifle made for 556 will shoot 223 as well but not the other way around. 223 rifle will not shoot 556. There is a difference in the dimensions. You need to look at more than the caliber
Not exactly. A rifle chambered in .223 WILL chamber and fire a 5.56 cartridge, but it can result in an unsafe over-pressure situation that can damage the weapon and even possibly harm the shooter.
OP: The chamber needs to be a 5.56 chamber, or .223 Wlyde (A match chamber specifically designed to handle 5.56 ammo). If you are specifically looking at AR-15s, it has been a LONG time since I saw one that did not have one of those two chambers cut into the barrel.
Also, it's the marking on the BARREL that matter, not what's on the lower.