Disclaimers; I Am Not A Lawyer (IANAL), any legal knowledge I may have is certainly at least second hand and possibly out of date, I have no vested interest in the subject, and the law is sometimes finally determined by judges who are often appointed for life and may well be senile and/or asleep when compelled to decide whether to ruin someone's life over some obscure legal point..

Nonetheless, with all respect, and not trying to be the least contentious, I'm not at all sure that this advice is entirely correct.

Please consider that innumerable companies and individuals are routinely manufacturing and selling muzzle-loading "firearms". They are often well over .50 caliber, often bear no resemblance to any historical piece, are usually not serial numbered, and are readily available without an FFL. They certainly use combustion to propel a projectile. The federal law defining "firearm" in this case seems to have something to do with "fixed ammunition". I doubt if a potato would qualify. Reenactors often procure and use cannon with bores measuring in inches, and I haven't heard that they needed to be serial numbered or registered.

You might also consider that, at least as of a few years ago, there were groups of people (presumably with adequate machining skills) who contended strongly that the laws permitted them (I was about to type, "they had the right", but that's another issue, really) to manufacture any firearm *for their own use* that it was otherwise legal for them to own, without serial numbers, registration, or FFL, and they were exercising that right, without legally "becoming" firearms manufacturers. The last I heard no persons with badges had apparently taken exception to that idea.

People, sometimes people with badges, tend to make assumptions about the law based on what they think is reasonable, but that just doesn't work, no matter which side of the issue you're on. I don't know much about "spud guns", but I doubt that the appropriate federal agencies would be much interested unless one is otherwise involved in a crime in some way.

As for property confiscations, a little investigation will show that there are thousands a week now without convictions... being "innocent" is no longer any protection there... but that's really an unrelated issue.