Do you have any spent brass that you have processed to the point of being ready to reload, but not reloaded yet? If so, what is the average resized length of a handfull of them? I ask because If for some incredibly odd reason, your die is sizing them longer and narrower, as in able to fit in the chamber with a shaving alongside, they will still pass the thunk test, but not fit when in battery without being shoved past the chamber lip. Also, if you hand cycle the round from the mag and ease it into the chamber, you should not be able to thumb it into battery using the forward assist and reasonable force, if it's too long. While I've never seen a set of dies do that, I have only used about 14 dies sets total.
hoot