Apples and oranges, but always infringements.
Still, automatically assuming that something is illegal is a fallacy here. It's proven to not be a destructive device, so the DD part of the NFA and AWB do not apply.
Can Cannons and Golf Ball launchers should be legal... They're not viewed as actual rifles or NFA Devices.
https://www.thetruthaboutguns.com/atf-approves-modified-x-products-can-cannon-no-longer-a-nfa-weapon/