You can make it too, with or without an FFL, just gotta pay the 200 dollar tax to make it along with filing all the paperwork and waiting while the ATF screws around with the paperwork, which can take like 6 months. Contrary to the beliefs of some, yes, it costs 200 dollars to make ALL NFA weapons, but it does indeed only cost 5 to transfer AOW's like these.
One thing I forgot to mention is that the alternative with less regulation is black powder pistol sized shotguns. They are available, and as I understand it do not even require background checks. The only downside is that the best capacity you'll likely find will be 2 shots, and that's not that great with a muzzle loader.
Edit to add: For the sake of clarity, if you make one of these yourself, you need a virgin receiver, not a receiver that was already on a non NFA shotgun, because that would be a sawed off by federal law. Same goes for the installation of a shoulder stock on it. Can't do it, because the feds consider that to also be a sawed off.