Right now I've made two diffrent types of spud guns one with spark plug as a means of ignation and the other with two bolts and a spark jumps across the gap they preforme about equaly, but the bolt type makes a bigger spark but then you have two holes in your compustion chamber so it's probaly more likley for the PVC pipe to crack or potentaly blow up.
Step 1: Assemble the compustion chamber
Glue the 8" length of 2" PVC to the 3" cleanout adapter. Do not glue the plug on to the adapter. Sand the PVC area of the PVC pipe that you will be gluing this will make the glue hold better. Next, you will need to drill the hole for the spark plug. The hole needs to be smaller than the spark plug so you can screw it into the PVC. the hole should be drilled 4" from the back of the combustion chamber. The spark plug gap needs to be streched out so it is almost straight.
Step 2: Assemble the barrel
Now you will glue the 2" PVC pipe to the 2" to 3" converter. Sand the PVC pipe agian and use plenty of glue. Glue the converter to the combustion chamber
Step 3: Electrical Assembly
Connect the electrical wires to the BBQ igniter and spark plug then wrap the electrical wires and connections with electrical tape. Then, tape the BBQ igniter to the combustion chamber with the duct tape.
Wait 24 hours before using the spudgun if you shoot it before it is dry the whole thing will blow apart.