A potato gun (also called a spud gun) is a simple projectile launcher that uses either combustion or compressed air to fire a potato out of a PVC pipe barrel. Combustion-powered designs are the most popular for beginners because they’re cheaper and easier to build, requiring little more than PVC pipe, cement, a fuel source, and an igniter. Here’s how both types work and what goes into building one.
Combustion vs. Pneumatic: Two Approaches
Combustion potato guns work by igniting a flammable gas inside a sealed chamber. The expanding gases push the potato down the barrel. Pneumatic designs use a tank of compressed air released by a valve. Both get the job done, but they differ in cost, complexity, and power.
Combustion guns are simpler. You spray fuel into a chamber, seal it, and hit an igniter. The trade-off is less consistent performance, since the fuel-to-air mixture inside the chamber changes with each shot. Pneumatic guns are more repeatable because you control the exact pressure, but they require an air compressor, a pressure-rated tank, and a fast-opening valve, all of which add cost and build complexity.
In terms of raw speed, fuel choice matters enormously for combustion designs. Testing by the Defense Technical Information Center found that acetylene produced an average muzzle velocity of 138 m/s (about 308 mph), far ahead of every other fuel. Methanol came in second at 48 m/s, while butane, ethanol, and propane clustered together between 28 and 35 m/s. Most backyard builders use propane (from a handheld camping cylinder) or butane (from a lighter refill can) because they’re cheap and widely available, even though they sit at the lower end of that performance range.
Materials You’ll Need
The core structure of a combustion potato gun is two sections of PVC pipe: a wider chamber where the fuel ignites, and a narrower barrel the potato travels through. A typical setup uses 4-inch Schedule 40 PVC for the chamber and 2-inch Schedule 40 PVC for the barrel, connected by a reducer coupling. You’ll also need:
- PVC fittings: A threaded cleanout adapter and cap for the back of the chamber (so you can open it to load fuel), a reducer bushing or coupling to connect the chamber to the barrel, and any elbows or extensions you want for your design.
- PVC primer and cement: These chemically weld the joints together. This is not optional. Friction-fit joints will blow apart under pressure.
- An ignition source: A piezoelectric igniter salvaged from a barbecue lighter is the standard choice. It generates a spark without batteries or external power.
- Two screws or bolts: These act as your spark gap inside the chamber. Mounted close together through the chamber wall, they let the spark jump between their tips when the igniter fires.
- Wire: To connect the igniter to your spark gap if they aren’t mounted right next to each other.
- Fuel: A can of propane, butane, or a similar aerosol propellant.
Schedule 40 PVC is rated for 280 PSI at 2-inch diameter and 220 PSI at 4-inch diameter (at 73°F, non-shock). Combustion pressures in a typical potato gun stay well below these ratings, but never use thinner-walled pipe like DWV (drain, waste, vent) PVC. It’s not pressure-rated and can shatter.
Building the Chamber
The chamber is where fuel mixes with air and ignites. Start with a length of 4-inch PVC pipe, roughly 12 to 18 inches long. Glue a threaded cleanout adapter onto one end using PVC primer and cement. This gives you a removable screw cap so you can open the chamber to spray in fuel. On the other end, glue a 4-inch to 2-inch reducer fitting that will connect to your barrel.
Before cementing anything, dry-fit all the pieces to make sure everything lines up. When you’re ready to glue, apply primer to both surfaces first (the outside of the pipe and the inside of the fitting), then apply cement and push the pieces together with a quarter turn. Hold for 30 seconds.
Here’s the part most people rush: curing time. For 1.5 to 3-inch joints, PVC cement needs at least 8 hours to reach full cure at pressures above 180 PSI. For 4 to 5-inch joints, that jumps to 12 hours. If temperatures are below 60°F, add even more time. Firing a potato gun with uncured joints is one of the fastest ways to split a fitting.
Installing the Ignition System
Drill two small holes in the chamber wall, about half an inch apart. Thread a bolt or screw into each hole so the tips protrude inside the chamber with a small gap between them. This is your spark gap. When the igniter sends voltage through, the spark jumps between the two metal tips and lights the fuel.
Take apart a piezoelectric barbecue igniter and you’ll find two wires inside. Extend each wire to reach your spark gap, connecting one wire to each bolt. Secure connections with electrical tape or solder. Some builders skip the bolts entirely and just use two nails or metal pins pushed through the chamber wall, spaced closely enough for a spark to jump. Either method works as long as the gap is small, roughly 3 to 5 millimeters.
You can also use a spark plug threaded into the chamber wall if you prefer a cleaner installation. Drill and tap a hole to fit the spark plug threads, then wire the igniter to the plug’s terminal and ground.
Attaching the Barrel
The barrel is a length of 2-inch PVC pipe, typically 3 to 4 feet long. Longer barrels give the expanding gases more time to accelerate the potato, but past a certain length the friction starts working against you. Three feet is a good starting point.
Cement the barrel into the reducer fitting on the front of your chamber. Make sure it seats fully. The inside edge of the barrel is what cuts your potato into a plug when you ram it in, so a clean, sharp edge helps. Some builders use a file or deburring tool to sharpen the inside lip of the barrel slightly.
Loading and Firing
To load, push a potato firmly against the muzzle (open end) of the barrel and ram it in with a dowel or broom handle. The barrel edge cuts the potato into a cylinder that fits snugly inside the pipe. Push it down until it sits just past the end of the barrel, near the chamber opening. A tight fit matters. If the potato slides loosely, gas will escape around it and you’ll get a weak shot or no shot at all.
Unscrew the rear cap, give a short spray of propane or butane into the chamber (one to two seconds is plenty), and screw the cap back on. Too much fuel actually kills performance because the mixture becomes too rich to ignite efficiently. You want a thin, even mix of fuel and air. Some builders give the gun a gentle shake to help the fuel disperse.
Point the barrel in a safe direction, away from people, animals, and anything breakable. Press the igniter. The fuel ignites, pressure spikes inside the chamber, and the potato launches out of the barrel.
Choosing Your Fuel
Propane and butane are the most common choices because they’re sold everywhere and easy to meter with a short spray. Propane, from small green camping cylinders, gives muzzle velocities around 28 m/s (about 63 mph). Butane, from lighter refill cans, performs slightly better at roughly 35 m/s. For most recreational use, either one works fine.
Methanol delivers nearly 50 m/s but is harder to handle since it’s a liquid at room temperature. You’d need to pour a small amount into the chamber and let it vaporize. Acetylene dramatically outperforms everything else at 138 m/s, but it’s also far more dangerous to store and handle, and it pushes chamber pressures much higher. For a basic PVC build, stick with propane or butane.
Legal Considerations
Potato guns occupy a legal gray area in the United States. Federal law defines a “destructive device” as any weapon that expels a projectile via explosive or propellant and has a bore diameter greater than half an inch. A 2-inch barrel is well over that threshold. However, the law also excludes devices that are not “designed or redesigned for use as a weapon,” which is the exemption most recreational potato guns fall under.
The practical reality is that enforcement varies widely. Some states and municipalities explicitly ban them. Others treat them as firearms. A few have no specific rules at all. Check your state and local laws before building one. Regardless of legality, never fire one toward people, buildings, roads, or animals. A potato traveling even 60 mph can cause serious injury.
Common Problems and Fixes
If your gun won’t fire, the most likely culprit is the fuel mixture. Too much fuel is actually more common than too little. Try using a shorter spray, or open the back cap for a moment to let some fuel escape and fresh air in, then reseal and try again. If the igniter clicks but produces no spark inside the chamber, check your wiring connections and make sure the gap between your metal contacts isn’t too wide.
Weak shots usually mean the potato isn’t sealing well inside the barrel. Use a fresh potato and make sure it’s large enough that the barrel has to cut through it, not just slide past it. Waxy potatoes like Yukon Golds tend to form better plugs than starchy russets, which can crumble.
If you notice any cracks in your PVC, stop using the gun immediately. PVC doesn’t deform before failing. It shatters, sending sharp fragments outward. Inspect all joints and pipe sections before each use, especially if the gun has been sitting in a hot car or in direct sunlight, both of which degrade PVC over time.