I haven't answered as I am having trouble answering.

These Rigby rifles scream big bore. But my mind says a big bore has two barrels. And if lacking two barrels a magazine. A single shot will do it, but could result in disaster. Hunting solo is far different to having one's hand held. No one to do one's killing for you, or stopping. I think the single shot expert DG hunter is a rare breed. Lots of practice is demanded. Muscle memory not thinking.

Some have mentioned carry a second scoped single shot rifle in addition to a DR in the same cartridge. For long shots. Assumes someone to carry stuff for you.

But I do like the idea of a sleek single shot as a plains game rifle. Capable of careful medium to longer range hunting shots on medium game.

For the first, seeing the rifle is a Rigby a .416 No.2 would make a lot of sense. Capable of medium to long ranged shots, at hunting distances. Powerful enough for any DG but only one shot.

A .375 H&H Flanged? A medium calibre single? Perhaps .333 Jeffery? A .318 WR? A .275 Mauser, rimless or rimmed. Particularly if the rifle was sleek and light. A "Rigby" cartridge again in a Rigby rifle.

A strong preference would be a .30 Flanged Super or .300 H&H Flanged.

The latter would mate well with a .450 DR.

Completely different if I had a Ferlach or German single, a hotrod 7mm SEvH of some description. Or a 6.5x68 or R. The rifle would need to be light and sleek with a long barrel.