As I posted earlier GENERALLY (rule of thumb) If it is marked 5.56 you can shoot either, if it is marked .223 shoot ONLY .223 in it. HOWEVER, as has been mentioned there are exceptions to every rule, Ruger's marked .223 are GENERALLY OK to shoot 5.56, and the .223 Wylde is chambered to take both. There may be others that are, like Ruger, marked .223 but can fire both. DO NOT ASSUME IT. If in doubt contact the Manufacturer before firing 5.56 in any .223 chambered arm.