A couple of Merkel .470's and one Krieghoff 9.3 mm is what I have personally seen. Most carry bolt actions but almost all have wished they could afford a nice double rifle.

One gentlemen who did several years PHing for Holland & Holland agency hunts had a staff rifle of a Holland & Holland sidelock .500/465, but unfortunately had to hand it back in with he left the outfitter. He said he was spoilt for life as a result. 'Churchill' also made him a .500 NE as "his" safari rifle but he never actually used it. The insurance they demanded on it was too high!