It works fine. Both .223 Remington and 5.56 NATO have the same new case dimensions, the .223 being the civilian version of the 5.56 NATO. If the brass is once-fired, though, and has been through a different gun, especially a full-auto weapon, it is not uncommon for it to be stretched to the point you need to use a small base die on it the first time you size it. Once it has been fired in your chamber, though, the standard die usually works. Once in awhile you get an AR that just has to have it go through a small base die every time, but that's an exception and it suggests the rifle is stretching brass pretty hard on extraction itself and might be a candidate for a smaller gas port.
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