A spud gun is a simple projectile launcher that uses either a burst of combustion or compressed air to fire a potato out of a PVC barrel. Most builds require about $30 to $60 in parts from a hardware store and can be assembled in an afternoon, though the joints need hours to cure before you can fire. The two main designs, combustion and pneumatic, differ significantly in complexity, reliability, and power.
Combustion vs. Pneumatic: Choosing a Design
Combustion spud guns ignite a flammable gas inside a sealed chamber to launch the potato. They’re simpler to build since there’s no air tank or valve system, but they’re less consistent. Each shot depends on getting the right fuel-to-air mixture, and misfires are common. Pneumatic guns use pre-pressurized air released by a valve, which makes them far more reliable and repeatable. If you want something quick and fun, combustion is the classic route. If you want consistent power and the ability to fine-tune your shots, pneumatic is the better choice.
A pneumatic build adds complexity: you need a pressure-rated air chamber, a ball valve or sprinkler valve to release the air, and a way to pressurize it (usually a bike pump or small compressor). A combustion build only needs a sealed chamber, a spark source, and a can of fuel.
Parts and Materials
Both designs share the same basic structure: a wide chamber connected to a narrower barrel. Here’s what you need for a standard combustion spud gun:
- Barrel: 2-inch diameter Schedule 40 PVC pipe, about 3 to 4 feet long
- Chamber: 3- or 4-inch diameter Schedule 40 PVC pipe, about 2 feet long
- Reducer coupling: connects the wider chamber to the narrower barrel
- End cap: seals the back of the chamber (a threaded cleanout adapter lets you open it for cleaning)
- Igniter: a piezoelectric grill igniter or lantern lighter
- PVC primer and cement: for permanently bonding joints
- Fuel: propane, butane, or hairspray
For a pneumatic build, replace the end cap and igniter with a Schrader valve (the same type found on bike tires) for filling and a ball valve or sprinkler valve between the chamber and barrel for releasing the air.
A Note on PVC Ratings
Schedule 40 PVC in the 3- to 4-inch range is rated for 220 to 260 PSI for water at room temperature. That sounds like plenty of headroom, but those ratings are for static water pressure, not the sudden force of combustion or compressed gas. PVC manufacturers explicitly state their pipe is not suitable for distribution of compressed air or gas. When PVC fails under gas pressure, it doesn’t split neatly. It shatters into sharp fragments. This is the single biggest safety concern with any spud gun build. Schedule 80 PVC has thicker walls and is a safer choice, especially for threaded connections. Manufacturers recommend against threading Schedule 40 pipe at all.
Chamber-to-Barrel Ratio
The volume relationship between your chamber and barrel is the most important design variable. The widely accepted sweet spot is a chamber-to-barrel volume ratio between 1.5:1 and 2:1. Many builders split the difference at around 1.75:1 or 1.8:1. Going from 1.5:1 up to 2:1 might gain you roughly 10% more muzzle velocity, but it also means a bigger, heavier gun. For a first build, aim for 1.5:1 and keep the whole thing manageable.
To calculate volume, remember that a wider pipe holds more air per inch of length. A 4-inch chamber that’s 18 inches long holds significantly more volume than a 2-inch barrel that’s 36 inches long, so the math works out even though the barrel is physically longer.
Assembling the Gun
Dry-fit all your pieces first. Slide the barrel into one end of the reducer coupling, attach the chamber to the other end, and cap the back. Once you’re happy with the fit, it’s time to cement.
PVC solvent cement works by chemically melting the surfaces of both pieces, fusing them into a single joint. Always apply PVC primer first to both the pipe exterior and the fitting interior. Then apply cement to both surfaces and push the pieces together with a quarter-turn twist. Hold for about 30 seconds. You can carefully handle the joint after about 5 minutes at room temperature for pipes in the 1.5- to 3-inch range, but don’t pressurize anything yet. Full cure for high-pressure applications takes at least 8 hours for pipes in that size range at temperatures between 60°F and 100°F. In cold or humid conditions, double that time. Rushing the cure is asking for a joint to blow apart under pressure.
For the igniter, drill a hole through the middle of the chamber wall sized to fit your piezoelectric lighter. Lantern lighters are compact and come with their own mounting hardware. The spark gap needs to be inside the chamber where fuel vapor will collect. Test the spark before you seal the end cap by clicking the igniter in a dark room. You should see a bright snap of electricity jumping the gap.
Choosing and Using Fuel
For combustion guns, your fuel choice matters more than most builders realize. A study testing five different propellants in the same cannon found dramatic differences in performance. Acetylene produced average muzzle velocities of 138 meters per second, far ahead of the pack. Butane averaged about 35 m/s, with ethanol and propane trailing close behind in the high 20s to low 30s.
For a backyard spud gun, though, practicality matters more than peak velocity. Propane (from a small camping canister) and butane (from a refillable lighter) are the easiest fuels to meter consistently. You can count seconds of gas flow to approximate the right amount. Hairspray works but is notoriously inconsistent because the spray contains varying amounts of actual propellant mixed with solvents and fragrances.
The key principle is getting the fuel-to-air mixture right. Too little fuel and it won’t ignite. Too much and you’ll smother the combustion. You want a roughly stoichiometric mixture, meaning just enough fuel to react with the available oxygen. In practice, this means a short 1- to 2-second spray of propane or butane into the chamber for a typical 3- to 4-inch diameter build. Start with less and work up. If you get a weak “whomp” with little power, add slightly more fuel next time. If you get nothing at all, you may have overfueled and need to fan out the chamber before trying again.
Loading and Firing
Cut your potato to slightly larger than the barrel’s inner diameter. Push it into the muzzle end of the barrel using a dowel or broom handle, ramming it down about 6 to 8 inches. The potato should fit snugly enough to create a seal but not so tight that it won’t move. Some builders use a sharpened barrel edge to cut a perfect plug just by pressing the barrel into a whole potato.
For a combustion gun: unscrew the rear cleanout cap, spray your fuel into the chamber, quickly screw the cap back on, aim, and click the igniter. For a pneumatic gun: close the release valve, pump air into the chamber to your desired pressure (start low, around 30 to 40 PSI), load the potato, aim, and open the valve sharply.
Cleaning and Maintenance
After several shots, combustion guns accumulate a yellow, gummy residue inside the chamber. This is mostly starch from the potato mixed with combustion byproducts, and it causes real problems. When enough gunk builds up, it can ground out your igniter’s spark, turning that bright blue snap into a weak orange flicker that won’t light anything.
Rubbing alcohol is the most effective cleaner for this residue. It dissolves the starch and combustion grime and evaporates quickly, which matters because you don’t want leftover liquid interfering with your next shot. Soap and hot water work too but take more scrubbing and leave moisture behind that needs to dry completely. A toilet brush with a flattened head fits well inside a 3- or 4-inch chamber. For the barrel, wrap a rag around a dowel. If you have a chamber fan (some builders install small fans to mix fuel and air), avoid getting water on it. WD-40 on a cloth works better for fan cleaning.
Clean your chamber every 10 to 15 shots, or whenever you notice weak sparks or misfires. A clean chamber produces a noticeably stronger ignition, and the difference between a gummed-up gun and a freshly cleaned one can be dramatic.