This is certainly not my invention and these arent even pics or videos of my guns (I have 3 of them but these are all pics I found online) and I know there have been topics about them before but they are such an awesome way to spend a few hours both building and then blasting away with that I thought I would share in case some of you have not had the good fortune to have learned of this miracle of pvc before. There are many variations of the spud gun, this is the most basic of designs that works very well and can be built in a short amount of time for less than $40.
The basic concept:
Items needed to build: (do not try this with anything other than schedule 40 PVC! Some serious pressure is built and ABS pipe will shatter possible causing harm) most of this can be found at a plumbing supply store, big box stores often do not carry 4" PVC.
1) Propane grill push button ignitor and some electrical wire to connect it to the nails/screws
2) 4" PVC Cleanout With Plug
3) 4" PVC Coupling
4) 4" X 1.5' PVC SCH40 Pipe
5) 4'' To 2" PVC Reducer Coupling
6) 2" X 4' PVC SCH40 Pipe
7) 2 Nails or long deck screws
8) PVC primer & cement
9) Basic hand tools
10) A rod or dowel long enough to push the spud into the barrel
11) Spuds (ammo) and propellant - anything from an aerosol can that is flammable will work but cheap hairspray seems to work the best for me.
Instructions:
Step 1 - Cut the PVC pipes with a hack saw to the appropriate lengths according to the diagram above. (Make sure the pipes are cleaned).
Step 2 - Hammer two nails or screw in 2 long deck screws across from each other into the 4" PVC pipe (Combustion Chamber) where shown so that the ends nearly touch (your spark from the piezo ignitor needs to arc here). Wrap the stripped ends of the igniter wire right below the head of the nail. Make sure the points of the nails are about a 1/4" away from each other inside the pipe. Push the igniter button to make sure that you see a spark inside the pipe.
Step 3 - Apply PVC primer then PVC cement to the ends of the pipes where shown while cementing all the pipes together - DO NOT cement the cleanout lid shut, you will be removing this lid to charge the chamber with propellant for each shot (see pic above).
Wait 24 Hours for the cement to dry before shooting (this is important).
While waiting you can chamfer the end of the barrel to make it easier to cut into the potato when loading, and tape up the ignitor into a position on the 4" pipe that is comfortable to press while holding the gun, and paint your gun or have it hydro dipped and attach optics if you like that sort of thing.
How To Use:
Shove a potato with a ramrod or a stick down to the end of the barrel, try to fill the entire circumference of the pipe with the potato, you want a tight fit. Spray your propellant into the combustion chamber for a quick second and quickly screw in the cleanout lid (less is better, too much propellant and you wont get a good burn). Aim the barrel downrange, push the ignitor button to shoot. If it doesnt fire you likely have too much fuel and not enough air, try opening the lid and blowing fresh air inside then do a shorter spray of fuel. I keep an old towel with me to wipe down the inside of the chamber after a few shots, it seems to help. You will also want to clean the threads of the cleanout and lid every so often, especially if using hairspray or they will gum up and get very difficult to open otherwise.
Much more info is available online including complete parts kits you can order and assemble as well as fully built guns. Google is your friend. Have Fun!!!