I made one, Pretty sick, Had a automatic bbq sparker ignition about 3 sparks every second pretty wicked, Blew it to peices with Methanol and shot that potato way too far.
I made one once out of a straight piece of PVC. It *usually* didnt work. I was running after a few buddies with it, and they were in my garage, one of them jumped behind a snowmobile, and one crouched in the fetal position and i blasted him with an ONION. He had about a 5" welt on his back and he wreaked of onion all weekend. I don't recommend shooting each other with them. And yes, the onion did bring some tears to his eyes.
I have a 1" barrel pneumatic made out of PVC. It has the sprinkler valve system, so you can shoot it remotely if you get long enough wires.
I shortened the barrel because it looked really strange, now it is WAY louder. Has quite a bit of kick to it as well.
I don't go above 75-80 psi and I covered it in electrical tape to protect from U.V and hopefully help if it explodes.
i riped the compressor of the back of a fridge buy making a stage pump out of one i reached 1800psi if ur interested go onto spud files and look for my user you can see mt posts and lots of cannons ive made
i made a gun to take 100 bar and i can put a berings thro a 2x4 with ease