Doc is absolutely correct! You can use your 5.56 Brass to load for your 223 rifle.
But you should never shoot military 5.56 ammunition in a rifle chambered for 223 Remington.
The reason is the 5.56 Military Chamber has more free bore (some call it lead) in the chamber. To simplify this is the distance of the chamber where the casing stops and the riflings begin. And taking in consideration the 5.56 NATO ammunition is also hotter than 223 Remington Ammunition. And the "true" 223 Chamber has less free bore (Lead) in the chamber than the 5.56. So if the 5.56 Military ammunition is fired in a true 223 Remington Chamber the pressure spikes to possible unsafe levels. So with that said and as Doc mentioned, you can use your 5.56 cases with lower powder loads. I would start at the middle of recommended 223 powder charges per the loading manuals.
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