Yes if its marked .223 it is dangerous. If its a bolt action marked .223 a 5.56 is totallly dangerous and overpowered. An M-4 which is a 5.56 but will take a .223 is a different situation.
While some of this is true, the important factor is head space in the barrel and the bolt. An improperly head spaced barrel/bolt for 5.56 is just as dangerous as a .223 with 5.56mm in it.
Bottom line is that any AR owner knows (or should) that they need to spend time measuring their parts if they assemble it themselves, check the head space, and the leade. Most completed guns are already head spaced in the factory and are set for 5.56.
Service parts are an issue that needs to be considered in an AR type rifle. A matched set barrel/bolt is significantly more expensive than either of the parts individually. Not that setting the headspace is that difficlut, and the gauge block is $15 or $20. Obviously, too long can be corrected (shaved to meet the spec), too short is dangerous and hard to determine even with the gauge.