Have you, by any chance, cleaned the barrels with a good, copper solvent?
I've only followed the Howa break-in process on one rifle, but do run some copper cleaner through my rifles periodically to check for buildup. Might only be one part of your puzzle, but stuff adds up. Have you checked the mount screws and tried swapping out a scope with a known good one? A .300 in particular can be hard on scopes and mounts, one reason our resident expert uses one for testing.
