All spud guns propel projectiles down their barrels using pressurised gas in the same manner as a firearm (although at a much lower pressure). There are three basic ways that spud guns may achieve this:
- By the combustion of a gaseous fuel-air mixture; this is generally referred to as a combustion launcher, and its pressure is limited primarily by the energy density of the fuel-air mixture (less than 100 psi (7 bar) with all safe fuels).
- By the release of compressed gas (normally air) through a valve; such a launcher is typically referred to as a pneumatic launcher, and its power is limited primarily by the pressure of the air supply, be that from a compressor, manual pump or bottled gas.
- By the combustion of a pre-pressurised fuel-air mixture; this is called a hybrid launcher, and yields higher pressures than that of a normal combustion spud gun, limited only by the construction of the launcher (generally a few hundred psi).
Combustion powered spud guns typically have the least complex designs, the four basic elements of which are:
- A fuel system
- A combustion chamber
- An ignition source
- A barrel
In order to fire, the operator loads a projectile into the barrel, adds fuel to the combustion chamber (for example aerosols or propane), and triggers the ignition source (often using a piezoelectric BBQ igniter). The fuel then ignites, creating hot expanding gases, and forcing the projectile out of the barrel. Distances vary greatly depending on many factors, including the type of fuel used, the efficiency of the fuel/air ratio, the combustion chamber/barrel ratio, and the flight characteristics of the projectile. Common distances vary from 100 to 200 metres, and some cannons exceed 500 metres of range.
Advanced combustion launchers may include metered propane injection to ensure proper fueling, chamber fans to mix the fuel with the air and accelerate venting of the chamber after firing, multiple spark gaps (spark strips) to decrease combustion time, and high-voltage ignition sources (flyback circuits, stun guns, camera flashes, etc.).
Combustion launchers are usually less powerful than their pneumatic or hybrid counterparts.
A large pneumatic design: The projectile is loaded in the muzzle. The air reservoir is filled to 120 PSI using the Schrader valve. Upon opening the solenoid valve, the air from the reservoir is transferred to the projectile, which is fired out of the muzzle.
Pneumatic launchers are considered a little more difficult to build due to the need of a completely airtight construction. These cannons have four basic components:
- A filling valve
- An air chamber
- A pressure release valve
- A barrel
In a pneumatic spud gun, air is pumped into the pressure chamber. After the desired chamber pressure is reached, the pressure release valve is opened, allowing the gas to expand down the barrel, propelling the projectile forwards.
The filling valve is usually a commonly available type such as a Schrader or Presta valve but other assemblies to pressurise the cannon such as quick release connections with ball or check valves have been used.
The pressure release valve is often one of a variety of commercially available types such as a plumbing ball valve, an irrigation sprinkler valve, or a quick exhaust valve. Experienced builders often make their own valves for this purpose to gain greater flow and faster actuation. The most common custom design used is the piston valve. Multiple valves arranged to be triggered together are occasionally used as an alternative to a single larger valve.
The range of pneumatic cannons is more variable than the range of combustion spud guns due to the increased variation possible in the components. Typical ranges are slightly higher because of the greater power, but the maximum range of some high power pneumatic cannons has been said to be over 1000 metres (1km).
Pneumatic spud guns are generally more powerful than combustion spud guns; a typical combustion gun generates chamber pressures of 10-70 PSI / 69 - 483 kP , while the average pneumatic gun can operate at pressures in the vicinity of 100 psi / 690 kP, or higher.