Back in the early 60s I took most of my weekly paper route earnings (50 cents) and bought the weapon from an older kid on the next block.
It was a piece of about 2-1/2 pipe welded to a base plate of 1/4 steel with some steel rod support straps. It looked like a WWII military morter. The bottom end had a plate welded on the end of the pipe that was tapped to accept a car spark plug. A couple of inches or so above the spark plug end was a crosspin rod welded on each end that served as a stop for our "bullets".
Well I just happened to have a model T Ford coil wired up with "rail road" issued 6 volt battery and a toggle switch (the trigger).
Propellant? Synasol for lighting the Jungers, rubbing alcohol from the medicine cabinet, and you guessed it all the lawn mower gas in the neighborhood! Gas didn't work real good at first. But we found out an ounce or so aerated with the hose from a tire pump worked swell.
Ammo; tennis balls, red rubber balls, basically any thing we could find and jam down the bore. If we would have thought of potatoes we would have used them too! We resorted to pulling off the covers of all the storm sewers around a three block area to find lost balls. The soggy ones had poor trajectory. Once we fired a ball it would go out of sight above the roof tops a city block away. We never found any after we had our "load" worked up.
Cops finally took it away from us. Best 50 cents I ever spent but I lost the model T Ford coil.
Randy