You may want a couple of buddies to go in buying the material with you, or make 3 at the same time, as the pipe comes in 10 foot chunks. I bought all material at a local "Home Depot" for about $15.
Note on choice of material: I have seen and heard of plans for spud guns using PVC pipe. In fact, an article in the February issue of "Modern Gun" uses PVC. I chose to use schedule 40 ABS plastic. The black pipe usually used for sewerage. If you want to know why I chose ABS, take a chunk of PVC pipe. Hit it with a 25 lb sledge hammer. It fragments into many *sharp* pieces. Try this with ABS. The sledge hammer bounces off the pipe and smashes into your foot. But it didn't break! (The pipe, that is, I don't know about your foot). PVC also gets brittle with exposure to sunlight. ABS just gets hot. Only ever use schedule 40! That's the thick stuff. It costs a little more, but not that much more. The bill of material says 10 foot lengths, only because that's as small a piece as is normally sold.
Step 1 - Cut the combustion chamber to size. Cut off a 14 inch section of the 3 inch diameter pipe. You don't need the rest of the 10 foot length, so save it for future bazookas, or make one with a couple of buddies splitting the cost.
Step 2 - glue the 3inch to 2 inch bushing into one side of the 3 inch coupling, glue the other side of the coupling to one end of the 14 inch combustion chamber. Make sure the joints are clean first and be liberal with the glue.
Step 3 - glue the threaded coupling to the other end of the combustion chamber (using the slip-joint side, obviously) make sure the glue doesn't run into the threads.
Step 4 - Cut the "barrel" to size. Cut off a 36 inch (3 foot) length of the 2 inch pipe. Glue this into the other end of the bushing you've glued to the combustion chamber. You should now have the complete gun, but it's not ready for firing just yet.
Step 5 - Using a file, taper the "muzzle" for the last half an inch on the outside. This will serve to cut the potato as it's rammed in.
Step 6 - You'll need to mount the sparker inside the end cap. If you got the Coleman one, it is threaded and has two nuts with it. There is also an angled piece of metal meant to hold the ignitor inside a lantern. Take the knurled knob off the end of the shaft. Be careful - there's an extra flint inside the knob. Unscrew the nut and discard the angled bit of metal. Drill a hole dead center in the ABS end cap of a diameter to take the shaft of the ignitor. Mount the ignitor inside the end cap, put the nut on the outside of the shaft and tighten until the ignitor is held in place. The shaft will slide back and forth, but won't come out. Put the end knob back on and tighten the lock screw.
Step 7 - Make sure the glue has "cured". I left mine overnight before firing.
Step 8 - make a ram rod. I used surplus 1/2 inch PVC pipe, 4 feet in length. A broomhandle, etc. will do. Measure and make a mark about 2 feet 8 inches down the ram rod.
To fire: remove end cap. Ram a potato from the muzzle end. The tapered end will cut the potato to size. Make sure it has a good seal as you ram it down with the ramrod. Ram to the mark you made. I've found most misfires happen when there are gaps between the potato and the barrel where gasses can escape. Spray 2 - 5 seconds worth of cheap hair spray (white rain, aqua net) I'd use an "unscented" one if you can, or the gun stinks after a few shots! Start at 2 seconds and build up! After the hair spray, quickly screw in the end cap. One twist of the ignitor knob sends the spud skyward!
You can get 3 shots off a big spud. Partially baked ones are fun - they seal in better and shoot farther, but they do break up and the barrel is a mess to clean up afterwards.
These things have a tendency to attract every 8-12 year old kid in the neighborhood.