All parts are white sch 40 PVC glued with wet/dry PVC solvent glue. The tapered 2" to 4" union is modified--I used a router to remove the lip inside of it so the 2" pipe can slide all the way through. The 2" pipe extending into the combustion chamber is perforated with a few 1/2" holes so that it won't interfere with combustion. The projectile must be pushed up to the front edge of the combustion chamber with a short ramrod. If you put the igniter in through the side of the combustion chamber instead of on the plug, then you can glue a short PVC pipe ramrod right into the square part of the plug and use it to push the projectile up to the correct position. This thing works well with any snug fitting ammo, including potatoes, onions, tangerines, apples, etc. Spoiled fruit & veggies don't go to waste at my house--they end up in orbit! A scoop of paintballs is pretty neat too--sort of like a monster shotgun--very effective out to 100 feet or so. My favorite projectile is still the water balloon.
To prevent the propellant from breaking the water balloon, I place it in a 9oz waxed paper Dixie cup, which acts much like the wad in a shotgun shell. I split the cup on two sides right down to the base, then I drop a small water balloon into it and fold the cup around it. Next, I push the water balloon into the breech, mouth first. On firing, the cup is stripped away by air resistance and falls to the ground within a few feet. The water balloon continues on at high speed.
The propellant is Aqua Net hairspray. A 4-second spray is usually sufficient--then quickly screw in the breech plug. In cold weather a bit more spray is needed, in hot weather a bit less. The report is a deep, throaty, BOOM!!! Warn the neighbors beforehand or you might get the cops called on you. After dark you'll be treated to a big bolt of orange flame when you shoot the thing. The muzzle velocity is amazing. Never shoot at people, pets, or cars--it's just too powerful for that even with a water balloon. High angle fire can lob a water balloon 200 yards with ease. Don't worry about the pipe breaking as long as you use only hairspray and air for propellant. I've fired mine hundreds of times with no signs of stress or failure. If you attempt to fortify your propellant with oxidizer or higher energy fuels, you're asking for trouble. It's a plastic gun--use your brain.
December 2003 UPDATE!
I've just completed a refined version of the breech-loading PVC gun. It incorporates an electric trigger that I liberated from an Aim N Flame disposable butane torch. A pair of wires carries the high voltage pulse back to a spark gap in the combustion chamber. The gun can be balanced over the shoulder and aimed like a bazooka. Performance is most satisfactory, with a truly obnoxious muzzle report and substantial recoil. The little lemon-size water balloon projectiles are practically supersonic! I've also changed my fuel. I now use unscented RIGHT GUARD deodorant spray--the original kind in the brown can. RIGHT GUARD gives performance similar to AQUA NET, but without the sticky residue. I have included a hastily thrown-together photo album of the new gun.Have a browse...