I have shot a few cape buffalo. I have used a 450 No2, a 450/400 3 1/4", and a 9,3x74R [the only scoped gun used on buff].
As you can tell I prefer the double rifle for buff [and elephant] hunting.

The only buff I have killed with one shot was with the 9,3 [go figure], but my wife did kill a buff with one shot of the 450/400.

After it is all said and done I think the BEST rifle for hunting cape buff is a 450/400 double with a scope in QD mounts.

With my scoped 400 I have taken warthog, baboon, and lion, with iron sights I have taken deer, pigs, caribou, buff, and elephant.

A scoped 400 double is a great choice for the modern sport hunter.

Might be best overall.