Potato cannon legality https://en.wikipedia.org/wiki/Potato_cannon_legality
in US: bare minimum it is a dangerous device as in pelletgun or archery equipment.
how/why its used, as well as ballistic features may cross a threshold.
if you construct a firearm, you must not have legal restrictions on ownership.
if the projectile of a firearm,is an actual .50calibre or greater,that is a destructive device and you must be rigorously vetted to construct or own that
He engineered his own gluelam support beams in his garage which could lift a car. He rewired his house using all stranded wire, bus bars and crimp connectors and it was all mil-spec.
It also says if you use compressed air that completely disqualifies it from "destructive device" potential classification in the eyes of the law.
"it must be muzzle loaded, not rifled, and not use any projectile which is intended to be used as a weapon."
compressed gas propulsion is not a firearm, unless it [gas] is produced by combustion or explosion of a chemical [incl. propane. ]
magazine or breech loading, rifled barrel, a cartridge holding projectile, and charge as part of design will seal the deal for definition as firearm.