The muzzle wall thickness can safely get smaller as the bore-size increases. The reason is that pressure is usually lower for the larger cartridges. The logical end-point is the shotgun.

BFJ: the thicknesses you quote would probably result in a very barrel-heavy double. Muzzles need to be quite trim to place the bulk of the rifle's weight back between the hands, and get the balance-point somewhere near the hinge-pin. Each barrel is stiffened by both the ribs and the second barrel, therefore they do not need to be thick-walled to prevent 'whip' or 'flip', or whatever you want to call barrel-flex during firing.

Judging by the PMs I get from potential home DG-builders, few American bolt-rifle smiths 'get' this, although it seems to be common knowledge amongst most Brit and a good many European gunsmiths.

Slim those muzzles right down, and you will get a lively, quick-handling double: the Holy Grail!