crl
The Above Posters have given you good advice...
However, there is no double rifle like a good British double rifle. Even Ray and Dougaboy will/should admit this.

[Do not make me quote y'all from our "personal" conversations

, I do not want to "drop a dime" on you, but I have a pocket full of change

].
In a Big Bore double, if you have the money and the time you can find a good one. Contact www.champlinarms.com
As to calibre. The 470 is the current modern favorite. Even Federal loads 4 different bullets for it. It is a great round.
I have a 450 No2. When I was shopping for a double I wanted a .458 bore so I could use 350 Hornadays for deer and pigs. I have been very, very happy with my 450 No2, especially in Africa where I have taken 2 buff and 3 elephants.
The truth is [as I see it

] any calibre double from the 450 to the 475 is equal in "killing power". I would just want it to be British if I had a choice.

PS I also have a British 450/400. [Mine is a 3 1/4", the 3" is just as good]. I would not hesitate to recommend the 450/400 as a persons first/ and or only double rifle.