Parts List: Fig. 1

  1. 36" length of 2" pipe (for barrel)
  2. 4"-2" reducer coupling
  3. 12" length of 4" pipe (for firing chamber)
  4. 4" clean-out (female, receives the end cap)
  5. 4" end cap
  6. 2" female threaded / slip joint (goes on the front of chamber)
  7. 2" male threaded / slip joint (goes on back of barrel)
  8. PVC or ABS cement (solvent)
  9. Hot glue gun and sticks

You may use PVC or ABS, I couldn't get the 4" in PVC easily so I used ABS and I kinda like black better for it anyway. The barrel doesn't have to be the same, though - I have 4 or 5 different barrels most of them ABS, but the 2 1/2" barrel is PVC.

The igniter may be a pushbutton-type piezio-electric barbeque igniter (expensive - about $15) or the flint-wheel (cheap - about $4) type (Coleman lantern igniters work well).
  For the piezio pushbutton type, you must make put a paper-clip or stiff wire from the base (like the ground on a spark plug) to in front of the business end. It will look sort of like a spark plug. (See Fig. 2.)

Now put some hot glue on the end cap so it only goes 1/3 or 1/2 way into the clean-out fitting. This hopefully will blow out if it gets overpressured and prevent the whole thing from blowing up (very, very bad) it the projectile gets stuck (a safety valve of sorts).

Assembly

  1. Cut all pieces to size.
  2. Temporarily place clean-out coupling on 4" pipe and mark a spot approx. 3" away from edge of fitting (on pipe). Now drill a hole for the igniter (check the igniter diameter and try to make it pretty tight) at the spot you just marked.
  3. Make sure you have the igniter working well before installing it. Insert igniter and install nut to hold it in place (if it has one). Seal with hot glue on inside and outside. Turn igniter so spark is visible through open end of pipe for visual test of igniter.
  4. Now mount clean-out, reducer, and barrel thread couplings on 4" pipe. Use plenty of cement.
  5. Bevel out end of barrel with file or whatever. (For automatic cutting of potatoes to size, just push it in there hard.)
  6. Blue non beveled end of barrel into male thread coupling.
  7. Wait for 15 minutes or so before trying it out.

Operation

  1. Open end cap.
  2. Insert projectile in end of barrel. Potatoes are the canonical projectile, however, apples, nerf balls, tennis balls (in a 2 1/2" barrel), maybe even a water balloon (but I think that would explode, if you try it, email me and tell me what happened).
  3. Tamp projectile into barrel with ram rod (make from broomstick of piece of scrap wood) until its about 4-6" away from end of the barrel (way down there - it goes a lot farther when it has more time with the expanding gases pushing it.
  4. Spray propellant into the rear of the gun, then immediatly cap, making sure to stop where the dried glue is.
  5. Point away from persons, press the igniter, and hang on!
  6. Before next shot, blow into the chamber to clear out the exhaust and bring in fresh air.

Just remember: be careful and have fun. If you have done anything interesting with your spud gun, e-mail me and tell me about it. This page made by Colin Bennett, email him if you like. I don't take responsibility for this page or ANYTHING that happens, anything you do is at YOUR OWN RISK, so you don't think you can blame me for whatever you chose to do with the spud gun.