How to build a pneumatic air cannon

[delusional_reality]

Interestingly enough I've actually made what you describe (except this was self contained firing all I had to do was flip a switch and watch) for my leaving certificate here in Ireland,
instead of potatoes it shot table tennis balls about 5 meters into the air at 45 degrees, (I subsequently had to make a deflector shield so that the balls would actually hit the table.


I used a compressor from school to constantly fill the chamber (I installed a pop up valve if the pressure got too high/someone left it unattended) and a 555 circuit to trigger the solenoid valve (with a diode and two V resistors you could change the timing) I also made a gravity fed magazine and an I.R sensor to indicate when the magazine was empty.

I should be collecting it from school soon and I'll make a video of it firing then but a pic is below:


http://i8.photobucket.com/albums/a30...aticy001-1.jpg
(this is what I originally envisioned, but found out the project would be over the size limit so I turned it on it's side)
http://i8.photobucket.com/albums/a30...ishy/zENDz.jpg

the orange acrylic box behind the gun was what housed the assembly and I was using a fire extinguisher with a Stabilizer Regulator (stolen from a paintball maker) to bring the pressure down to acceptable safe levels,

the deflector wasn't made at the time of the photo and I was still in the tweaking ''Will it work'' stage.
I got an A1 for it and my written exam

but I'll stop boasting now and help you out,

you say you want an auto, why don't you use a pop up valve to connect to your barrel, so that your compressed gas source fills the chamber to say...100PSI... and when it hits that pressure your pop up valve gates the gas pressure into your barrel, thus firing your projectile, (you can buy pop up valves for various pressures) what do you think?