Use ABS pipe or at the bare minimum, pressure rated PVC. ABS is better because IF it does explode, it just cracks and the pressure shoots out the crack, instead of into a million sharp pieces like PVC. Buy some ABS/PVC sealant, put a small amount around all the ends of pipes that will be glued to other pipes. Buy an electric grill sparker, the generic kind with the red button. Then you will just have two wires that you need to drill two small holes into the ignition chamber to thread them through. Circumcise the ends of the two wires so that they are exposed metal wires and give them about a quarter inch gap between them. When you click the red button you'll see a good sized strong spark. If you don't see a spark, bend them closer. Use something cheap like Aquanet hairspray, axe body spray or similar. It will be plenty powerful enough to launch a potato a quarter mile or more. Wait 24 hours for the ABS/PVC sealant to cure and test fire it with socks first. The further you stuff your projectile down the barrel, the more powerful, high pressure, and louder it will be. So DO NOT start with a potato shoved all the way down the barrel, that shit will sound like a 12 gauge and possibly explode if you fucked up construction. What's fun is a golf ball inside a sock, or rocks in a sock.
Unscrew the back end cap, spray for about 1 second long, screw the end cap back on quickly and press the red button.
>t. Have made over 10 potato guns in size from larger than the OP image to small handguns all using the same method