I figure the bullet used is equal to or more important than the calibre. I too think 416 and up calibre wise but a 9.3 or better yet a 375 H&H with Hydros or similar.

Bolt or double. I like the handling of a good balance double but would rather a bolt for DG. I have carried an SLR in the Army Reserves, 10ld of rifle. I have also carried a Bren Gun & M60 18+lbs. after a couple of hours the heavier arms take their toll. Also, from an old African hand I knew, there are as many dead men because they had a double as their are because they didn't use a double. Doubles are far less tolerant of abuse, sand, dust etc than a bolt action.