I am new to double rifles (don't own one yet) but it seems to me that if you want a double rifle that will perform the same as a 30/06, that 7.62x54r is the obvious choice. It's one of the oldest modern rimmed cartridges still produced. Both hunting and military ammuntion has is still manufactured. Alternatively 8x56r might do the trick too. This round is also still manufactured (although in small volume).