To do what the OP described would require first getting a NFA tax stamp to manufacture an AOW (Any Other Weapon). It would be a $200 tax stamp, involving the same process as in making a shortbarrreled shotgun, shortbarreled rifle, suppressor, destructive device, etc.
AOW's include "smoothbore pistols" along with gadget guns and other items.
You would need a Form 1 for manufacturing, in duplicate, some passport sized photos, fingerprints, a CLEO sign-off, and the affidavit form. However, before all of that - you would have to make sure the gun would be legal in your home state. Some states classify smoothbore pistols as "shortbarreled shotguns" which then some of those states prohibit shortbarreled shotguns.