I doubt you would get in much trouble if you use a little common sense and just use hair spray for propellent. I build this one about 20 years ago and its been in the back of my shop ever since. I had a guy working on my pond and he mentioned his son had just built a potato gun and that made me dig mine back out. I will say that when you fire it you get a pretty good "thump" noise. I fired it a couple of weeks ago using the usual 1-2 second of spray and it sounded like a 22 mag. and I never saw the potato come out but it did. Supersonic? I don't know but it sure sounded like it.
NRA Endowdment Life Member.....F&AM
Curio and Relics License Holder (FFL 03)
Gun confiscation will not happen in my lifetime.
"Of all tyrannies, a tyranny sincerely exercised for the good of its victims may be the most oppressive. It would be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated; but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." C.S. Lewis