Dangerous advice. If a gun is marked .223, best to ask the manufacturer. While most of the time they do interchange, the max pressures for 5.56 are almost 8,000 psi higher. Some guns, such as a certain model of the Ruger Mini 14, some competition model, is NOT setup to shoot 5.56 and can ONLY shoot .223. Save a gun, life, or limb, call the manufacturer.