Not a traditional spud gun, but my father (who has been a welder for his whole life) built a couple differently sized cannons out of aluminum (one fired wine corks and the other was about tennis ball-sized). We used acetylene gas. Honestly kind of terrifying and incredibly dangerous, not recommended. They only come out on independence day.
We built one using acetylene as the source gas generated from calcium carbide and water, ignited by a spark gap from an electric fence. The combustion chamber was a 40 gallon oil drum and the barrel was a 20’ scaffold pole.
Round 1 worked a treat, firing a champagne cork sized wooden baton far away into the distance.
Round 2 was a disaster. Calcium carbide needs water to react into acetylene. Our method involved putting a few pints of water into the oil drum (on its side), then a few rocks of calcium carbide, then the igniter, and finally the scaffold pole resting under it’s own weight in the small, off centre oil drum aperture.
On this second round the end of the pole was accidentally submerged in the puddle and the wooden baton got jammed. This meant the explosion forced water up the gun barrel — a pole just resting there in the mouth of the oil drum — launching the entire pole down the garden. It must have gone a good 50 feet or so. This is a steel javelin weighing a good 30lbs.
Round 3 ruptured the oil drum. It sounded like a bomb going off.
