This ignition based gun is good, although the spark is probably inferior to a taser based ignition. See how to make a high-quality potato launcher, otherwise known as the spud gun, or spud cannon, or potato cannon, or mashed potatoes maker, or potato gun, or whatever else it may be called, all with parts from the local hardware store. The instructions go by quick, but are definitely thorough, and you'll find out that this potato launcher is strong, sturdy, and easy to make, not to mention fun to shoot (at George Bush).
4" to 2" Reducer
2' of 2" PVC Pipe
Lantern Spark Igniter
1 Can of Starter Fluid
(These are all ABS-DWV Schedule 40 PVC fittings)