‘Think artillery’ when it comes to the 4-bore double rifles of Austrian gunmaker Peter Hofer.
If he's still in business, there's a gunsmith (Ken Owen) in Moscow, Tennessee still making 4-bore double rifles. His "in-house" ammunition drives a 2000-grain bullet @1600 fps. They're all customs, of course, but they run about 25-lbs to help tame the recoil. He mostly makes Nitro Express guns but you can tell he draws customers with thick wallets because he once built a 4-bore with woolly mammoth ivory inlays. IIRC, that one was featured in Guns&Ammo. If you see one of his 4-bores on auction at Rock Island or elsewhere, they typically fetch north of $100k ... used.
Wow, wow! The 4 and 8 bore rifles are awesome, indeed. 300 - 400 ft/lb of recoil buddy, that will get your attention. And the guns are art work.