I often use CCI 400 in .223 --- Bet your brass is LC (Lake City). In that case, they could be crimped in and they will not fit. You will need to swage the primer pocket to get most any type primer to fit. I would suspect that they measure smaller because of the depriming stage ...
Aight I'll add that to my list for a third order from midway USA I got some lc on the way but I'm working with some that has FC above primer and 06 below. Not a hundred percent on the 06 but that's what it looks like.
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