450D

Plenty of poop for a Buffalo and adequate for Elephant. You need to pick up a copy of John Taylors 'African Rifles and Cartridges' check Amazon for a good price. Also get a copy of Graham Wrights 'Loading for the British Double vol 2'.

This caliber was a real favorite and probably the most popular of all the Double Rifle calibers. It combines handling and power with low felt recoil and plenty of snot for Plainsgame and DG.

Make sure your dealer tells you what kind of action it has and get an inspection period. I insist on a minimum of 5 days and not 3 because of the weather at times. If you can get another opinion of a knowledgeable person when you have it. For the money a couple of hour drive is not a bad investment.

Tell him you want to shoot it and get at least 4 rounds of ammo. Shoot two rights and 2 lefts at different targets to see how each barrel regulates. Each barrel should regulate a 1-1.5 inch group. Don't worry too much about how far apart they are but as stated vertical can be a problem if it is more than about 1.5-2" at 50 yards.

Make sure he sends along the case if it has one or a fiberglass takedown case at least.