I think I'm a member, but I'm not distinguished or anything like that. I don't have my double rifle yet but after asking double rifle owners a million questions, I've come to the conclusion that the .470 is the best chambering to get and if you want anything else, the only other one to really consider is a .450/400 3" NE.

Personally from a standpoint of actual use I'd prefer the .450/400 3", but I think I'm getting a .470, because:

1) the .470 is the only double rifle cartridge loaded by a major ammunition manufacturer;
2) multiple sources of brass;
3) ability to use commonly available .475 Linebaugh bullets for reduced loads (if you're in the USA);
4) virtually all rifle bullets in the appropriate diameter are designed specifically for a .470 NE.
5) as far as i can tell. the .470 NE cartridge in actual use is at least five times as popular as any other double rifle cartridge...why I don't know, but because of that, brass is far less likely to be discontinued.
6) Butch Searcy will build a .470 with a spare set of 12 gauge shottie barrels

Richard