H&H .600 nitro express. I've never fired one and have no desire to do so, although my hand would be forced by a charging elephant. I've read that the recoil on a .30-06 is 12-14 foot pounds; the .600 produces 250 foot pounds of recoil.You really think you could bring yourself to torch one of these off Glen? And if you miss with the first barrel, you've always got the second - although being trampled or gored might be less painful.
I would definitely fire it - at the range - but would leave the hunting of dangerous game to better men like you. :)
Bakal, a Russian company, made a 45-70 double that was sold by Remington several years ago. That would be a neat rifle to have.