I'd like to talk for a minute about power sources and such. This seems kind of obvious to me, but there are a lot of people out there who just don't get it.
The schedule 40 PVC pipe is rated to 280 psi. Most sprinkler valves are rated to around 110 or 120 psi.
I commonly get asked by people how to hook up a CO2 tank to this. CO2 tanks have pressures as high as 1200psi. Do the math people! 1200psi in a system rated for 110 psi means PVC shrapnel tearing the skin from your bones in a rather ugly explosion!
The simplest and safest method for charging an air cannon is to use a bicycle pump that has a pressure gauge. It takes a minute or so to charge it up. The air cannon's air chamber is good for one shot. For paintball use, I treat the air cannon like a one shot device. I carry it around to somewhere near a flag station, wait, use it, then leave it 'till after the game. It's most practical that way.
Another charging method is to use an air compressor with a pressure gauge. A friend of mine had one that ran off the cigarette lighter connection in his car. It re-charged the air cannon quickly and easily between games in the parking lot.
The only way CO2 can be used is with a regulated gas supply. This means using a regulator on the CO2 supply to bring the output pressure down to 100psi, and using a safety valve to blow out in case the regulator fails and overpressures the air chamber. NEVER, and I mean NEVER hook CO2 directly into the system. I have gotten e-mail from people asking if they could just install a valve, and open it "real quick" so that just enough gas got through to reach 100 psi. The problem with that is that the tiniest pinhole leak, or failure to totally shut the valve, means that the pressure in the air chamber will slowly continue to rise until it is enough to burst, and cause serious injury or death. I have heard an unconfirmed report from a reliable source about someone losing an arm from a system like this.
High pressure compressed air (SCUBA tanks etc.): The same thing as CO2. Regulation and safety valves are essential. Don't even think about messing with this stuff if unless you are very experienced working with and designing systems for high pressure compressed and cryogenic gasses.
Over all, if you are just building an air cannon as a fun toy, or for paintball, stick with the bike pump. It's cheap, safe, easy, and convenient (I've run regulated CO2, and it's added weight and impracticality).