@SHGYJ If your question was for understanding, rather than for "how to make a double rifle out of a shotgun" let me give you a bit of advice.
Of the gun makers you rattled off, the one that fits closest to the "British Standard" of proper geometry, balance, weight, and action widths is going to be Heym. They have a smallbore double that can handle tiny stuff up to 375HH (maybe 450-400 as well), a "standard" that can handle your 375HH up to 500NE, and the "Gross Box" that is in 500NE and 577NE. They are not making shotguns on these actions, they are "Real" double rifle actions. However, you can pay another $6000ish on a custom order to add a pair of 20 gauge barrels to your double rifle. It's a miserable shotgun, poorly balanced and overweight, but very handy for practice at home and even better for hunting birds on a safari. Nonetheless, its a double rifle with shotgun barrels added, not the other way around.
Heym is sort of the "ground floor" in purpose built, incredibly reliable double rifles. They hold their values better than any other maker outside of the UK as well. They are so well built they can handle heavy loads and unfriendly monometal solids.
While my heart leans British in fine guns, my head knows that the Heym is the epitome of the price-to-quality ratio.
