Do you mean 5.56? If so, they're very similar but slight differences. A 5.56 barrel can shoot a .223 but I wouldn't but a 5.56 in a dedicated .223 unless it's a .223 Wylde.
Like Mr. Evaniuck said if you're referring to 5.56, it's very similar to .223 but often has a higher chamber pressure. Typically if a barrel is stamped 5.56 NATO or .223 Wylde, it will be safe to shoot either 5.56 or .223 through the gun. However, if it is stamped .223 Rem, it is probably not safe to chamber and fire 5.56 rounds through it. Hopefully that helps.
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