223 and 5.56 are the same caliber. The very same. Like Brian said, the only diff is that 5.56 is a military name for this cartridge and it's loaded to a higher pressure. That said, knowledgable people on this forum have said that shooting 5.56 in a 223 labeled rifle is fine. That doesn't make it true, but I personally believe it. Anyway, if your son is reloading and staying below book maximum on powder charges, he'll be fine (unless he unwisely screws something up). He needs to do his homework very carefully on the reloading process.
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