Kevin Robertson of "Perfect shot" and his latest book "Africas most Dangerous" has no problem with a client using a single shot on dangerous game.
Thatis because it is your first shot that really is supposed to do the animal in and the client has a PH besides him with a stopping rifle to sort things out for when everything goes wrong.
Also a single shot like a Ruger no.1 can be reloaded very fast if a spare shell is kept handy.