Aleko,
I have to agree with Dugaboy, 400 Nitro and the others that recommend the 500/416 over the 416 Rigby for a double.
Even though I have 2 rimless drillings, and a rimless K 95 that have worked perfectly, for a big bore that will be used for elephant and buff up close I would stay with a rimmed cartridge.

I have shot one buff at 12 yards, one elephant at 12 yards and 3 elephants at 6 yards or under.
At those distances you want to leave nothing to chance.

However I would still like to have a 375 H&H double.... I think, just because I have so much 375 ammo and brass. However I have found my 9,3 Chapuis to be PERFECT, so I guess I will leave the 375 to my Blaser R 93.