You mean, I know something you don't Steve???? I'm shocked.
I'm not a lawyer and don't even pretend to play one on the internet, so if any of this is in error I apologize and you should check with the BATFE for clarification. Here's the story from ATF from the top of my head. In a muzzleloader it doesn't matter whether it is rifled or not, nor does bore size matter to ATF. When it comes to Cartridge pistols, ATF says that any thing over a .500 bore is a destructive device, doesn't matter whether it is rifled or not. You can apply for an Any Other Weapon permit(AOW) I don't remember off the top of my head what the tax for that is, but they get their money if you want to build one. The ATF says it must be rifled if it is shorter than the overall standards for a shotgun(18" barrel length, 26" overall),no smoothbores under the shotgun standard. The ATF also says that it can only fire one barrel at a time from one pull of the trigger, that doesn't mean you can't pull both triggers at the same time, but it must only fire one barrel from a single pull of the trigger, otherwise you have a machinegun and that requires a $200 tax and a whole different set of paper work. In a howdah configuration the legal limit in the u.S.A. would be .500 caliber, with either individual triggers or with a selective trigger and it must have the minimum rifling that would make it classify as a pistol. It must also meet all of the NFA and GCA requirements for a pistol.

The best example I can give of meeting the minimum requirements without paying the taxes is the companies that make a .45LC/.410 side by side derringer. They are under the .500 restriction and the last inch of the barrels are rifled making them a legal pistol. They will chamber and fire a 2 3/4" .410 shot shell but they are technically a pistol not a short barreled shotgun.

I believe there is probably a way around the ATF regs but the cost of production and the marketability would be limited. The ATF says that any action even a cartridge gun that is a pre-1898 is an antique and may be copied and doesn't fall under the GCA or NFA regulations, but it must be an exact copy of the action and it must fire an obsolete cartridge that isn't currently available through a commercial supplier in the u.S.A. and it cannot be a rimfire cartridge. Basically you would have to copy an action style that is easily proven to be a pre1898 design, it must be an exact replica and you could only chamber it in cartridges that aren't available commercially. Which presents the problem of even if the manufacturing cost didn't kill the idea, where does your customer get ammo? Kinda limits the market to handloaders only.

As to the rifled 12 gauge, they are defined by ATF as for sporting purposes only and are exempt, that is why some 12 gauges like the streetsweeper are considered a destructive device and are illegal(not sporting), while a 12 gauge Mossberg 500 with fully rifled barrel is legal(sporting). It's really just arbitrarily up to the BATFE Tech Branch.

And all that is worth what you paid for it, get it in writing for the BATFE if you plan to build something like a modern Howdah in a cartridge gun.

Colin