I think this is an interesting question. The only double I have owned was Rigby made in 1912, chambered for the .350 #2 which is similar to a .338 Winchester in terms of bullet weight and velocity achieved but uses .358 caiber bullets.
Another double I handloaded for was a Westley Richards in .360/400 caliber which was a little less powerful and used .366 (9.3mm) bullets.
Something in that range would certainly be wonderful for timber hunting bear, moose, elk, or deer, but when I think of outfitting yourself from scratch, I think I would aim a little heavier in power, paying attention to retain lightness for the rifle itself and also retain moderate recoil.
In that regard, the two cartridges that stand out to me are the 9.3x74 based on the prolific (or nearly so)components which would encourage both practice and use, or the .450/400 which was made specifically for doubles.
If.....you ever go to Africa, as fininancial circumstances do tend to change in life, then the heavier caliber that you have learned to shoot well in the US would be an ideal companion for bushvelt hunting for anything from antelope up to buffalo.
John
