I just made my first pneumatic air cannon, which was intended to shoot tennis balls, but it doesn't really work. In the past week, when I was planning and designing the cannon, I checked out some designs on the web but didn't copy any designs exactally. I went for a U shaped cannon with a 2 ft long 2 inch PVC air chamber with a sprinkler valve leading to a 3 inch diameter barrel. I couldn't find 2.5 inch pipe in the store, so I went with 3 inch, hoping it would work. The 3 inch pipe turned out to be too large for tennis ball, but even when I use objects which fit snug in the pipe the cannon just makes loud noises and doesn't shoot them far.
There could be an obvious design flaw that I can't figure out, or it could be an issue with my sprinkler valve. I'm using a Hunter brand valve, and it was just what I had laying around. My next step to diagnosing the issue is to switch to a different brand valve (one that is easy to modify to work non-electronically).
Anyway, here's pictures of the cannon with a small explanation for each:

View of the full cannon.

I use an air compressor and nozzle fitting to add air to the pressure chamber.

To activate the cannon, I just connect the sprinkler valve to a 9 volt battery. I have a switch that I want to install in the loose pipe, but I didn't install that as the cannon isn't working at all.
I have taken a video of me shooting a soft small toy soccer ball. It is the best thing I have found in my house which fits in the barrel, and it fits snug. Here's a picture of it in the barrel and below that is a video of it being fired at 40 PSI.

[youtube][/youtube]
Thanks to anyone that replies with ideas on how to fix this. I haven't tried anything above 40 PSI, but I can't imagine it would help that much to go up to higher pressures. I don't know if this is common or not, but it makes extremely loud noises when I fire (like a train horn deal). Is this a sign the valve is not letting enough air through, or what?