somthing to look at might be the port clearances on the muzzle brake.
I have recently become aware of an accuracy problem that develops in some braked rifles, the leading edge of the baffle bore holes, erode from the forces of the gas jet. As this happens it roles the edge into the clearance hole, causing the clearance to tighten up and create accuracy losses that are otherwise unexplainable.
Inspect the port holes very carefully, it will be obvious with a careful inspection if this is the problem. The quick fix is a brass rod with a split in it, wrapped with 320 grit emry cloth, and spun in a drill, as you polish off the burs. Once this bur is removed, it rarly comes back because the 90 degree edge that roled in is now removed from the erosion and the subsequent polishing.
Before doing this protect your crown with a mechanical barrier so you don't hit it with the abrasive.
I have seen this on a couple guns that came in the shop and then recently had one of my brakes do the same thing. I now have been sending out all my brakes for heat treat, hoping this helps the situation if the brakes are up to 32-34 rockwell.