I certainly am no Doubles expert but if the existing platform is a concern then a new rifle manufacturor could redo the package with a shorter fatter 450/416 round or a longer 450/416 round to prevent lawyer shopping because of an ammo screwup. If not then just go with the current 450/400 &411 bullets and a well thought out rifle.
If the current round is satisfactory then maybe a Double made like 400 NE referred to on a previous thread with a 470 reciever and appropriate barrels to give fine "between the hands" balance and lighter weight( vs a "heavy Double")should be manufactured.
Sounds like a Double a lot of folks would like to buy new, including me!!
-------------------- An armed man is a citizen of his country, an unarmed man just a subject.