I am very happy to hear you are getting exactly what you wanted in your rifle.
From looking at your groups I would say that playing with some seating depths a bit could possibly tighten up those groups as it appears most of your group variation is vertical variation in the group.
This is generally a result of velocity spreads at the ranges you are shooting and farther.
Still, With a 300 RUM getting sub 1/3 moa is acceptible at 322 yards [img]/ubbthreads/images/graemlins/grin.gif[/img]!!
The brake by the way is a Holland QD Muzzle Brake
Thanks for the report and have fun with your new long range hammer, 1/4 mile just does not seem that difficult anymore does it [img]/ubbthreads/images/graemlins/grin.gif[/img]!!!