With the advent of the vast array of saboted rounds for 12 and 20 ga shotguns, and realizing that they shoot very well, is it not possible to simply replace the barrels on a existing 20 ga gun and have a very servisable double rifle?

Yes I understand the dificulty of regulation but would not the whole project be easy except for the regulation. And you'd have a gun, assuming a 20 ga, that aproximates a .444 marlin or similar?