I think you said these were fl resized. It could be that you’re generating too much headspace or short base to shoulder length and the primers are backing out a bit and then being riveted by the case head as it backs up to the breech face. Try backing off on the charge a bit and just neck sizing the cases such that it takes a slight effort to chamber them. The cases and primers from the other mild loads fired on the same day seem to be giving a similar look so neck sizing or partial fl resizing might do it. I had this problem with my second 243 even with starting loads. Look at the primers after depriming a few cases. From the side, if they look like little mushrooms, you may have found your problem.
Velocities are a bit hi but part of that could be barrel length as someone mentioned. Several angles to consider.
I FL resized because most of my 257 brass is fired from a Ruger M77UL which I neck size only with the Lee Collet die. I have some new RP and Nosler cases. I'm going to back off some and start again with new brass. Then neck size only for this Ruger No.1 and work up again. Can't hurt.
