Originally Posted by OP
As to the signs of overpressure, two primers were completely ejected from the primer pocket.
I just went back to read that. You are waaaaaay
overpressure ... 80,000psi range.
It may be any number of things, from internal case volume differences to case neck interference
in the throat, to bullet seating depth, ...but back off at least 1½ to 2 full grains with that
Verify that the cases are trimmed to min-length spec (<2.1" ?)
...and see if you can scounge up a chronograph