Shoulder stocks and hand guns. BATF does not consider a BP cap and ball weapon to be a firearm. Go figure. It's legal to mail, ship, send, haul or drag one anywhere. No barrel length restrictions of any sort. For all practical situations, you can do anything you want with it. It isn't a firearm.
Now, add a cartridge cylinder and the rules change. It is now a firearm and ALL applicable laws governing firearms apply. So long as the cartridge cylinder is in the gun, you MAY NOT add a butt stock unless your barrel is 16+ inches. Get caught with a butt stock on a short "firearm" and your looking at a federal firearms conviction, jail time, fine and loss of your firearms privileges for LIFE!! BATF does not take prisoners!!!
Additionally, if you CONVERT a BP handgun to cartridge, by modifying the frame, you have manufactured a firearm without a license. You may do this for yourself legally, but once the conversion is done, you may never legally sell or transfer said firearm. Modify a frame to accept a Kirst = yours forever. Re-cut the frame for a Richards style recoil plate and cylinder (home brew or Kirst) = yours forever. Cutting a frame for cartridge loading (Kirst without loading gate) and you have still manufactured a firearm = yours forever.
Coffinmaker