Interesting, What's the coal that you are using with the RMR 124gn? I just got in my 1st shipment of their new 124gn MPR JHP and on my Springfield 5.25 XDM competition I was touching lands at around 1.080 so I backed off .02 and loaded them with a coal of 1.06 and at 5.3gn of BE-86 the primers were getting real flat so I didn't even shoot the 5.4gn BE-86 test rounds.
It could be the primers as well as I've been using mostly CCI and Federal but lately the only thing I could find close by was Winchester WSP's that I hadn't used before and I'm thinking they are softer and show up pressure signs way before the other primers. The Alliant site shows max at 5.9gn with a 124JHP using a coal of 1.12 so 5.3gns of BE-86 seems way low to be seeing flat primers.