Re: Custom fitting a case to a factory chamber
step one: sell the RUM brass that you don't need.
step two: just purchase 300wsm brass (Norma if possible or win).
Either set up your fl sizing die according to your chamber or neck size them after they are fired first in your chamber. Rum brass isn't exceptional so it doesn't pay to go through the trouble since Norma makes excellent wsm brass.
I used to re-load but now I "hand-load".
-- Well, at least I try