Ignition
A lot of what people worry about a little too much for combustion cannons is ignition. A while back people were going out and buying stunguns under the sole presumption that it would make their cannon somehow more powerful than a cannon utilizing a simple piezoelectronic grill igniter.
The truth is, a single, very small spark will ignite a homogenous and stoichiometric mixture of fuel and air with no problems, and the same as a continuous or otherwise repeating spark that is somehow "hotter".
The differences in ignition when it comes to practicality are in performance (spark layout) and convenience. While a stungun won't magically lend more kinetic energy to your spuds, it does allow a bit more design flexibility than a simple grill igniter (such as safety switches, ease of operation, etc.).
A summary of Igniters
There are many ways to ignite a fuel mixture, from the simplest to the most complex, we have the following (safe) devices.
Flint Lantern Lighters
The simplest. These are designed to be mounted on camp fuel lanterns, and provide a more convenient alternative to lighting with matches. They consist of a hollow shaft with a keyed shaft running through the center, connected to a rough steel wheel at one side, and a knob at the other. A piece of flint is held against the steel wheel by a right angled bracket. The knob is turned, which rubs the wheel against the flint, creating a shower of sparks.
The mounting hardware designed to bolt through the match hole on a lantern has 3/8" threads which can be used to thread into a launcher. Simply drill a 1/4" hole and thread the nut in with some vise grips.
Piezoelectronic Grill Igniter
These are arguably the most readily available way to ignite both combustions and hybrids. They work by striking a quartz crystal with a small, spring-loaded hammer. This creates a voltage (potential difference) across the surface of the crystal of a sufficiently high voltage to easily jump up to 1/2" air gaps.
The big (gas grill) type are available at about $9 from any store which sells grills (Wal-Mart, Home Depot, Lowes, Menards, etc.), and the small, dinky kind (which can jump at most 1/4" in air) are available in any lighter marked as "electronic" (Aim n' Flames, Bic Electronics, etc.). The one you choose is based on your spark layout and preferences, along with space constraints.
Electronic Grill Igniter
Notice the lack of "piezo" in the name. These use simple electronics to charge and discharge and high voltage capacitor through a coil, which creates enough voltage to jump comparable gaps to a piezoelectronic igniter.
They usually run off of a AA or AAA battery. The big bricks that you can buy at Wal-Mart or Home Depot are very large, run off a AA, and are completed potted, restricting what you can do with them and how small you can get your circuits. However, if you check eBay you will usually find smaller models which run off of a AAA and only have the PCB itself potted, which allows for minimization by removing the battery holder.
Stunguns
Can be either very simple or very frustrating to use. Very cheap stunguns come with perfboard circuits which are prone to overheating and giving off a lot of radio noise.
However, moderate to expensive stunguns come with potted circuits which I have been unable to break as of yet. I would always just take a ride to your local sporting goods store to pick up a ~$30 stungun, which will yield a clean, small, and diverse potted circuit for your convenience.
One thing to note is that the circuits usually come with VERY small leads on the high voltage side, which require some careful stripping and soldering to extend.
Due to their relatively high output voltages and simple incorporation into larger circuits, stunguns are often used on more advanced combustions with multiple spark gaps, although it is more for convenience than gains in performance.
Homebrew Circuits
I won't go too in-depth here. These can range from modified camera flash circuits to 555 ignition coil drivers to CRT transformers.
I wouldn't suggest any of these for spudgun ignition unless you know what you are doing and like a challenge every now and then...
Spark Layout
This is about the only factor you need to consider for performance.
A lot of people generalize it as being "the more spark gaps in your chamber, the better." To a point, this is true, but it gets a tiny bit deeper.
It's important to understand how flame fronts burn, that is, in 3 dimensions. A flame front will expand in all directions from the spark until it meets an obstacle, such as the chamber walls. Also, from some simple geometry, we know that a flame front traveling in all directions (spherically) progresses faster than a flame front travelling in one direction (laminar).
So, we'll take a look at a simple combustion launcher with one spark gap at the back of the chamber. After ignition, the flame front will quickly meet the chamber walls and progress towards the front of the chamber, in laminar mode.
However, by placing that single spark gap in the very center of the chamber, the flame front will travel in all directions until it meets the chamber walls, and then travel towards the back and the front of the launcher, creating two flame fronts.
So, how do you burn the most fuel in the least amount of time? The answer is to divide your chamber into sections of d length, where d is the chamber diameter. In the center of each of these sections should be a spark gap. This way, you create multiple spherical flame fronts that burnt out when they meet an adjacent flame front or the chamber's walls, with little fuel consumed by laminar flame fronts.
This can be done in multiple ways, but the easiest way is to create a spark strip that runs down the center of the chamber.
So What Do You Suggest?
I suggest you quit worrying about ignition and start focusing on fuel metering and mixture fans. After this, I would suggest building a spark strip like mentioned above and using a piezoelectronic grill igniter to power it.