I am more of an admirer of the Short Double Barrel Shotguns like you see in "Roadwarrior" or "Outlaw Dove the Outlaw Years"; than I am Howdah Pistols per se...

However, one day after examining one of the Remington 870 "Witness Protection" Shotguns at tha local gunstore, I had an epiphany...

A short barelled Pump; Double Barrel; Lever Action--whatever--would be perfectly legal if it had a rifled barrel; and it wasn't over .50 Caliber...

One Minor Caveat: You have to start with a virgin reciever--once a long Gun; always a long Gun--legally speaking.

.45-70 works out very well. It should do a fair job of simulating 12 Gauge Recoil. Think about it--payloads are roughly the same, velocity is roughly the same. .45-70 would have some torque introduced from the spin created by rifling. If you build your pistol to 12 Gauge scale; the thicker .45-70 walls will make your Gun Somewhat heavier. (This is probably a good thing...)

At any reasonable range, the .45-70 should approach 12 Gauge stopping power (At least the SP you'd get from a foot-long barrel).

Idea number (2) Chamber your pistol for .500 Smith & Wesson Magnum--gets you a wee bit closer to 12 Bore...

Idea number (3) make a pistol to 20 Gauge outside dimensions; and chamber it for the .375 Winchester...

Idea number (4) If you want more "Shotgun-Like" performance--chamber your pistol for some of the .50-140 Brass that you see. Use token Rifling and a screw-in choke.

28 Gauge is .55 Caliber--so frustratingly too large to "Rifle/.410" it...

But the 3 1/4 inch .50 with noticably more shot--and regretably--a much longer shot column--should be roughly comparable. The really annoying thing is not being able to use 28 Gauge Shotcups...

Maybe you could persuade a small plastic factory to run you off a few thousand .50/ 140 shotcups--or improvise them from Aluminum foil; or thin Sheet brass...

Anyway, Colin and I have discussed the idea at some length. I don't have a Lathe at present--and can't imagine getting two Foot-Long .45 Caliber barrel blanks both chambered and turned to the proper countour without a Lathe--though I'm sure someone can...

RVM45