I know that the parent case/inspiration for the 300rum is the 404, so how difficult would it be to form down from the RWS or Norma parent brass?

The rim diameter is smaller on the RUM so you would have to use a lathe to cutdown the rim on each 404 case. Not real practical sense you can easily buy 300 RUM brass and 404 brass is not cheap to begin with. Regards, Dale

Alot more difficult than buying 300 RUM cases.
You could buy the brass, neck it down to .30 cal, and fire-form to get rid of the double shoulder, but since the base is .001+" smaller than your chamber, it will lay in the bottom, and concentricity would pretty much be impossible.
I'm sure a mzchinist coul come up with a jig suing a sizing die, but geez, that's a lot of work when the Jeffery cases are harder to come by, and more expensive, than the RUM.

Little more involved than I had hoped. I was looking for something more consistent than the remington brass I have been working with,..but oh well. Prep work will continue with the rem brass for now.

I was thinking if the opposite could be feasible, like usingn RUM brass in 460 G&A, OR 425 WESTLEY RICHARDS, or 404 JEFFREY, leaving the bolt face adjusted tothe the RUM base size. Has anibody done this? Thanks.