I am going to use a CCI 400, as it has never been necessary to use a magnum primer in any of my ar's. Yes the cup is thicker on the magnum primers, but it is not necessary. I've loaded 1,000's with regular ol BR4's as well and that is not a magnum primer.
FWIW, CCI BR4s and 450s cups have the same thickness. 400s are thinner. Doesnt mean they won't work though.