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.223 is a fixed commercial load since the mid 60s. 5.56 is military and is loaded at higher pressures. You can fire .223 in 5.56 barrels as the casing have the same dimensions but don't fire 5.56 in .223 barrels or you risk blowing your face off.
 
You're good to go , test a few rounds and look for signs of over pressure , elevated chamber pressures are revealed by overly flattened primers or by powder stains around the primer that reveal leaking gasses , also if the bolt seems harder to work , if none of that happens , fire away .

You're more likely to have a issue with a out of spec round of ammo , be it .223rem or 5.56 NATO , then you are using 5.56 in a .223 , A bad round is a bad round .
 
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