We used to make them out of beer cans and use lighter fluid for propellant.
I think Chuck is still in CA, and this state, in its infinite wisdom, has made it a crime to use a combustion-propelled spud gun. Compressed air is OK, but the old school hairspray guns have disappeared.
I'm pretty sure that it is the BATF that frowns mightily on potato guns that are fueled by combustible products such as hair spray (lacquer and thinner). Perhaps a replica pre-revolutionary flintlock black powder potato gun would be legal though.
If making potato guns illegal seems petty, think about this...When we were kids, one of my Dad's neighbors called the cops on us because we were flinging apples with a stick. Actually, he thought it was his civic duty to call the cops on us about twice a week. For some reason, he reminded me of our own Gladys Kravitz, aka old colonel.
On this pneumatic potato gun, it might not be a bad idea to incorporate a pressure relief valve set to a safe working pressure, even though most air compressors won't exceed 180 psi. I'd imagine that British potato guns are subject to proof laws.
