There are many ways to ignite a fuel/air mixture. Anybody who's anybody who's ever looked into accidental fires knows this.
There are several reliable ways to ignite the mixture inside of a spudgun. We'll list them in order of complication.
Arguably the most crude and simple way of igniting a spudgun, and possibly responsible for the most burns on people's fingers, hands, and arms.
The construction is simple: drill a small (anywhere from 1/8" to 1/4") hole in the cannon's combustion chamber. After fueling, touch a match or long-nosed lighter to the hole, and leaking fuel/air will ignite and the flame will travel into the chamber, igniting the rest of the mix.
Aside from a very small decrease in performance due to pressure loss, this is mildly dangerous as the pressure increase in the chamber will create a jet of hot gases a respectable distance from the touch-hole, capable of burning skin quite severely.
For the above reasons, it's not something I'd recommend.
These are one of the oldest methods along with piezoelectric igniters for igniting a spud gun.
They are flint igniters designed to be mounted inside of a propane or camp fuel lantern. By drilling a 1/4" hole through the chamber (a double layer of piping/fitting preferably), the igniter can be disassembled and threaded into the hole, and reassembled to result in the knob outside of the cannon and the flint and file-wheel inside.
AdvancedSpuds has a decent page on installation. In addition, I've included a simple diagram below:
These come in a number of sizes and shapes. By far the most commonly used are the big "red button" types used on propane BBQ grills for ignition.
However, they can be pulled from a number of butane lighters, ranging from long-nosed lighters for lighting grills and fires to "electronic" Bic's.
Each has two contacts. When the button is pressed, a spring is compressed until it is released after a certain pressure. This strikes a small quartz crystal with a hammer, generating a large voltage across it's face, which is conducted to the two electrodes on the exterior of the igniter.
Small igniters for use in butane lighters can generally jump up to a 1/4" air gap at atmospheric pressure. The larger ones used for grills may extend this up to 1/2" but a good rule of thumb is to use a single gap totalling no more than 1/4", or multiple gaps equalling the same.
These are circuits based on a voltage multiplier and capacitive energy storage to generate several pulses of high voltage (up to 50,000 for most mediocre stunguns), capable of jumping relatively large air gaps.
Stunguns can be bought in varying qualities. Reliable ones will have a completely potted circuit which simply get supplied with ~6 volts and outputs anywhere from 50 kV and up. Cheaper ones will have a board with components soldered on it, and a small air-gap which controls the capacitor's discharge through the output transformer.
Electronic igniters are usually more expensive than their piezoelectric counterparts, and bulkier as well, with completely potted circuits for the most part.
little circuits (similar) have been flooding the hobby, and are
extremely cheap and
decently reliable, providing a
high voltage output for a 5V input.
I won't go into too much detail here, but many people enjoy integrating their electronics hobby with spudgunning. Car ignition coils, flyback transformers, and even hand-wound transformers all find themselves jammed into project boxes or clamped to a cannon for ignition.
The simplest ignition coil driver is a switch, a small capacitor, and the ignition coil itself, along with a 12V source. Going up from there, drivers can be built using an astable multivibrator such as a 555 timer for switching voltage to the coil repeatedly, or drivers with feedback utilizing hall-effect sensors to switch current through the primary side of the coil at the most opportune time.
Camera flash circuits can be utilized either by themselves or in conjunction with an output transformer such as an ignition coil or flyback transformer. The simplest way to use one of these boards is to mimic the flash tube (in open air) inside the chamber by having three screws converge very closely together, wiring two to the leads of the capacitor on the board, and the third to the output of the trigger transformer on the board. The high voltage pulse from the trigger ionizes the air between the other two screws, allowing the relatively low (~300V) voltage of the capacitor to discharge across the gap, resulting in a very hot spark.