Specifications
The heart of the beast are the valves. This is an electrically triggered cannon using Hunter HPV valves found at some local garden care centers or at places like this:
Valves
Notice Hunter makes a cheaper valve, the SRV, which is rated to 150psi. I went with the HPV because I thought I wanted more, but I've been playing it safe with 90psi (I put it around a corner and pushed it to 130psi the first time and let it sit to make sure I wasn't "real close" and since then I've stayed at 90psi).
Get a battery box from Radio Shack and a couple switches (I use one flip switch for the safety, a push button for the trigger). Wire it like this:
Wiring
Get some
SHEDULE 40 or HIGHER PVC pipe. Schedule 60 is better. You MUST have this type of PVC or it won't hold the pressure.
Pick your barrel size first and then make sure your pressure chamber size is at least 2x as large. My design could be improved a lot since there are several bends between my chamber (bottom) and the barrel (top). In a perfect world would it would be in-line with the barrel directly in front of the chamber. If you can find a valve or PVC pieces that neck down then go this way. My design was built by going to Lowes with the valves and finding what pieces were in stock to do it. We originally wanted a 4" chamber and 2" barrel, but they didn't carry the reducers to do it......or we didn't have the mind muscle to figure it out. Here's what it came to:
1 3" chamber pipe. Around 50" long. Cap the front of it and drill it to accept an air valve. A valve with an integral pressure gauge is nice. Don't just thread it into the pipe. Put a washer and nut on the inside for strength.
At the rear of the pressure chamber we have a "T" fitting that accepts 3" along the plane of the barrel, but with the vertical port being 1.5".
I ran this 1.5" port up into a "T" that accepts 1.5" and splits it into 2 1" exits. I then used 1" elbows on both sides to make the turn to the 2 solenoids (which happen to be 1" ....bigger would have been better, but more expensive). After the solenoids I used 2 more 1" elbows to turn the pipe back into a "T" that is 1" in all directions. This runs into a "T" that has 1" on top and 2 "1.5" exits. This connects to barrel. I could have capped the end of the barrel but used a ball valve isnstead to help with cleaning and loading.
1 1.5" Barrel pipe. 50".
Use PVC primer and then PVC glue to seal it all together.
LET IT DRY 24 HOURS.
If not it WILL blow on you.
Start with LOW pressure and work your way up. Slowly. You are building a gun and load capable of hurting you or others. Don't get slopy and follow all rules of safe handling. I recommend stopping well below the range of your rated pipe. PVC standards are not to be totally trusted. Your gun may differ, but the risks are yours.
Feel free to e mail me for any problems/suggestions.
