Pretty much what others mentioned...To my knowledge the 5.56 casing is thicker than the .223 casing, which, in turn, creates more pressure. While a 5.56 chamber is built to handle the higher pressure, a .223 chamber is not. Because if this, you can shoot both 5.56 and .223 rounds in a gun chambered in 5.56. However, you do not want to shoot 5.56 rounds in a .223 chamber, as it is not built to handle the higher pressure, and can blow up your rifle.
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