As the unofficial PoA spokesman for flying Rednecks, allow me to offer some things I've learned in tater gub makin'.
Materials
At least schedule 40 PVC. Schedule 80 is preferred, but not necessary if you stick to less volatile propellants. Stay away from standard wall PVC.
Trigger Mechanisms
Generally speaking, any gas grill igniter will work fine. I find the battery powered ones offer more reliable spark, but the piezo igniters do offer a somewhat more satisfying spring resistence followed by a nice *click* whe it goes bang. The very best trigger is a stun gun with the leads positioned do the spark goes across the diameter of the combustion chamber.
Propellant
For recreational use, hairspray is quite adequate, readily available, and inexpensive. I prefer Aquanet.
*Do not, I repeat, do not use propane or acetylene! Even schedule 80 can't handle the pressures. Please allow my own independent research be a warning.
Projectiles
Taters work really well, offer a very tight seal, and provide a satisfying splatter, and can be bought cheaply and in large quantity. These are short range projectiles with unpredictable accuracy, and offen come from together in flight.
Epoxy castings are efficient, offer heavier weight and higher impact energy, and can be easily shaped to provide a low ballistics coefficient. The weight does affect trajectory, but hits hard.
Golf balls. They are made to fly efficiently, and offer the greatest range of any projectiles I've tried. Care must be taken to select an appropriate barrell diameter, as the projectile cannot be modified to fit your barrell. Also, you can close your stance and roll your right hand slightly over the top to achieve a draw if you need to bend the trajectory.
Specialty Projectiles
For indoor fun, marshmollows may be used.
Hope this was helpful.