Spud guns rule
Several years ago, I built one out of PVC, powered by a gas grill sparker hooked to 12g wires that arc inside the chamber. I have shot walnuts through sheet metal with it using cheap hairspray as fuel. The most explosive fuels that I have used were genuine ether starting fluid, followed closely by Ban deoderant powder spray. Apples, onions, chunks of watermelon or pumpkin, etc. are all good projectiles. I beveled the inside of the barrel end to get a tighter plug, and I used to lube the barrel with dish soap.
Here's the kind of power that you can get with a spud gun -
While I was out on a boat one time with some other zeros, I did the following:
Shot a potato up into the air
Put the gun down
Stepped over to the cooler
Opened a barley pop
Slugged down half of it in one gulp
Then, in the middle of a huge belch, the potato splashed down about 50 feet away.
I'm kind of afraid to shoot it anymore. I have thought about wrapping the body with several layers of fiberglass packing tape to help contain a backfire. I have also thought about using the black schedule 80 - much stronger. Metal pipe would be good, but it would be heavy. Also, I couldn't use my sparker ignition system because it would ground out on the gun body.
But then, I could just f***ing grow up too, I guess.
I used to be a bigger a-hole than I am now, but I think can still hold my own!