This is the design of my newer potato guns. They feature threaded barrel and chamber, making it possible to interchange barrels and chamber for interesting combos. This design is slightly more expensive being that it requires more parts but is still under 20 dollars.
A length of PVC for the barrel, 2-4 feet long, 2 inches in diameter
A length of PVC for the chamber, 8-12 in long, 3 inches in diameter
A male threaded adapter, 2 inch dia
A female threaded adapter, 2 in dia
A 3-2 in reducer
A female threaded adaper, 3 in dia
A threaded cleanout plug, 3 in dia
A gas grill ignitor
1. Glue the barrel section to the male threaded adapter.
2. Glue the reducer to one side of the chamber piece and the female threaded adapter (3in) to the other.
3. Glue the female threaded adapter (2in) to the other end of the reducer. This will accept the barrel.
4. Make the ignitor by drilling a hole in the bottom of the cleanout plug and sticking the ignitor through it.
5. After letting this dry you just screw the barrel to the chamber and then load and fire.