Your typical first gun

Before doing anything, it's in your best interest to read my safety precautions that you ought to take when working with this device. When you set out to building this gun you may think you're being sneaky. You may think you can go to the store and buy these components and nobody will know what it is. You may think you can go to various stores and get away with it. The truth is, unless you live in a large city that doesn't know about this stuff yet, you can expect to hear SOMEONE yell potato gun behind you in line at the checkout. Take my word for it.

I specify PVC piping although you may want to use ABS piping. It is argued which is better. I have a section discussing the pros and cons of ABS vs PVC farther ahead.

Materials

• 1 10 foot piece of 2 inch diameter schedule 40 ABS pipe
• 1 10 foot piece of 3 inch diameter schedule 40 ABS pipe
• 1 3 inch to 2 inch reducing bushing
• 1 3 inch coupling
• 1 3 inch threaded (one side) coupling
• 1 3 inch threaded end-cap
• 1 can ABS solvent-weld pipe glue. NEVER USE PVC GLUE on ABS!!!
• 1 Coleman sparker - these are easily found in any sporting goods store that has a decent camping section. They are normally made for putting inside a Coleman lantern or stove so you don't need matches.

Assembly

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 broom handle, etc. will do. Measure and make a mark about 2 feet 8 inches down the ram rod.

So, your gun is all assembled and you're ready to fire it for the first time. Good! You'll need to decide what propellant you will be using. If you're unfamiliar with the common ones, I have a section on propellants you may wish to read.

Your first shot isn't always a successful one. A handful of things can prevent the gun from firing. If it doesn't fire, read ahead to my troubleshooting segment.

Firing the gun is simple enough. Because you beveled the tip of the barrel, your potato is going to be cut to the perfect size as you press it down. When you're pressing the potato into the barrel, take your time. Keep it lined up so it's not at an awkward angle (which would give you a poor seal). Use the ramrod to slide your spud as far down as you can.

Spud in place, the next step is to fill the combustion chamber with your propellant of choice. Every propellant has a different fuel/air ratio, and it varies even more depending on the size of your gun. My suggestion: start low. Use only a little of whatever you've got, because you won't need much. If it doesn't fire, then put a little more in next time and slowly work your way up. As soon as it will fire, keep slowly increasing the amount of propellant and notice how the power of the shot increases. At a certain point, you will notice the power of the shots begin to decrease as you add more propellant. The point where you stop gaining power from shots is the optimum fuel/air mixture. Stick with that for the best shots!

Note: When you fire the gun, byproduct gasses (and liquids) may be formed by combusting your propellant, in the same way a car emits exhaust. In between shots, you should blow through your gun to flush those useless gasses out.
Note: I prefer to fill my combustion chamber with propellant first and then ram the potato. This compresses the gas in the chamber, increasing its volatility and the power of the shot.

Having a little trouble? Don't worry, it happens to everyone. Fortunately, there are only a few reasons why your gun should ever not fire reliably. And here they are:

Too little propellant

Ok, so it sounds too simple, but it can be the case. Not enough propellant in your chamber and it just won't fire.

Too much propellant

This is the more likely of the two cases. You don't need so much propellant that you could level a small city. If you keep trying to fire and it's not shooting, try less propellant. This happens to me constantly.

Poor seal

Poor seal? Yes, the potato (or other projectile) needs to fit very snugly in your barrel. Hold the gun up and look through it. If you can see light at the other end of your barrel, it won't work. For a shot to be successful, pressure will have to build up rapidly in your chamber. It will never build up at all if the gas can just slide by the potato.

Extreme temperature

This isn't so much a case of too much heat, rather too much cold. Gas will lose its heat extremely fast. This makes cold-weather shots difficult if not impossible. My propane will not reliably combust under 50'F. To fire in colder weather, keep your propellant as warm as possible, but not hot!. You won't be happy if your propellant starts to combust in its container. Just keep it warm, and it should still be able to fire in the cold.

Dirty/wet ignitor

Electric ignitors will tend to gunk a little at their contacts. You need to keep the two contacts as clean as possible on your electric for it to spark. A flint sparker won't work when it's wet, but is impervious to gunk. If you get your sparker wet, use a lighter to dry it off.

Too much exhaust

When you combust your propellant, other uncombustable gasses (and liquids) will be formed as byproducts. Every few shots, you should blow air through the gun to flush these gasses out. Just like in your car, if the exhaust isn't pushed out of the engine it will kill the performance.

It just won't work

There are days, few and far between, when the universe aligns itself against you and the gun simply isn't going to fire. This seems to happen whenever you're showing the gun to friends who've never seen it and expect to see spuds flying thousands of yards. My recommendation for this one: give up and try later.

Still having trouble?