Amazing difference just in the primers, Neale & with a full grain more powder with the BR4's. Those 'disked' primers almost let go & actually look like what a weak firing pin spring does. a bit of cratering from a large hole, as on most CZ 527's is slightly different as that shows a full dtrike, with flow. Pushed back material like that is either weak cups or weak spings. In this case, a week cup as you surmised and proved.

CCI Small Rifle Magnum primers (450's) have the thicker jackets as well as do RP 7 1/2's. Those three are the same and thickest of all. The Remingtons were developed for the .17Rem which was hard on primers of the 1960's. The 7 1/2's were the response and CCI followed suit with the BR4's and SML Rfl. Mags.

Nice performance form that hot core and the little 6.5.