Your primer piercing/blanking would probably have continued with even moderate pressure loads. It sounds as if you have blanked many primers in the rifle. If you would have looked at the tip of the firing pin after the first couple of blankings, you would have stopped and thrown the pin away as it was probably severely jagged and eroded from gas cutting, thus the continual blanking at even moderate velocities. This can also be exacerbated by a weak firing pin spring. The Gre-Tan solution is the proper approach for a Remington in BRX. I fireformed 300 cases with 30 gr. Varget and 107 Sierras and never lost a single case, or pierced a single primer. using Remington 7 1/2 primers. My barrel is an 8" twist, .236" bore Pacnor with .104" freebore. I used the false shoulder method along with jamming the bullet as I believe it keeps the base of the case against the bolt and does not stretch the case at the web. As for powders, Reloder 15 works well with similar charges to Varget but provides a little more velocity, but, my favorite for the BRX is Vhitavouri N140. I keep going back to it and get the best grouping and good velocity.
Ed
Ed