The RUM/H&H actions are getting hard to find and expensive. The magazine/action will still need modified to feed the heavy bullets seated long and the RUM case fits a standard magnum bolt face.
My latest project started as a Stevens in 300WM and is now a 338/300 RUM. I assembled it, cost was <$1000 for parts including the donor rifle. 28" Shilen barrel, NSS recoil lug, SSS trigger, B&C Medalist stock, locally manufactured/installed brake.