I have noticed the occasional bulged case heads when shooting Winchester white box .223 in my AR-15s cambered in 5..56mm.Doesnt seem to happen when i use surplus 5.56mm in those same rifles.
I also have a AR-15 cambered in .223 that the Winchester brass comes out in good shape when used.
My theory is the brass you have was fired from a 5.56mm chamber and the slightly bigger chamber allows this to occur.But i may be talking out my arse so take it with a grain of salt.
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