5. In-line Sprinkler Valve (1 in)
6. Compressed air blowgun with a 1/4 in NPT air connection
7. 18-in long PVC pipe (D 3 in)
8. PVC end-cap (3 in)
9. 1/4 in NPT air tank valve
10. 1/4 in NPT 0-60 psi pressure gauge
11. 1/4 in NPT brass hex or regular nipple. 1/ block strapping tape.
12. Strapping tape
13. Aluminium tape
14. And of course, ping-pong balls!
The first step we need to do is to modify the sprinkler valve so that it is useable in our assembly. You need to disassemble it and drill a 7/16 inch hole at the center of the top. Now you insert a 1/4-in NPT tap into the handle and cut screws in the hole you just made. Now you need to apply thread sealant to one end of the 1/4 in dia 1 in long brass nipple and place it in the tapped hole you have made before. Now you apply thread sealant on the other end of the nipple and insert it into the 1/4 in NP opening of the blowgun. You would also need to block the solenoid air-hole. Insert a sheet metal and cover it with epoxy.
The next step is to make the pressure reservoir. Before you make it, I want you to look at the diagram very carefully. Now make a 7/16 hole in the center of the 3-inch PVC endcap for the Schrader valve and the 3 in Dia pipe for the pressure gauge. Now use 1/4 NPT pipe to cut 1/4 threads in the hole as well. Now apply the sealing and insert the gauge into the hole and the tank valve in the end cap. Now assemble the reservoir as shown in the diagram and follow each step carefully. Now wrap the pressure reservoir with strapping tape.
Now that the basic components are in shape, we can begin assembling the cannon. Now you screw the modified pipe assembly into the 1-inch female pipe thread opening on the reservoir. Now you screw the valve assembly into the 1-inch female pipe-thread opening and used sealant to prevent leaks as well. Now you can use the compressor to pressurize air to 15 psi. Check the pressure valve for any possible leaks and take mitigation where necessary. Now test the gun by pressing the trigger and releasing the air.
Be mindful to follow all the safety procedures while working on this project because this kind of pressure is dangerous enough to cause significant damage to the human body. Only use ping-pong balls for this project and dont go beyond 15 psi as the reservoir may not be able to last under more pressure.
Good luck building one!