I would recommend reading up on what the ATF considers a "destructive device" or "any other weapon" before home building any cannons or similar things.
An ignorant but technically motivated person can also "accidentally" build an NFA form 1 or form 4 firearm by making a short barrelled rifle without the appropriate tax stamp, just by combining pieces together. Or by doing something like putting a vertical foregrip on something that is legally a pistol. Or by making a home-made silencer. And so on.
https://www.google.com/search?client=firefox-b-d&q=atf+defin...
An ignorant but technically motivated person can also "accidentally" build an NFA form 1 or form 4 firearm by making a short barrelled rifle without the appropriate tax stamp, just by combining pieces together. Or by doing something like putting a vertical foregrip on something that is legally a pistol. Or by making a home-made silencer. And so on.