Would this be it?
http://www.wired.com/images_blogs/underwire/2010/02/d9_gas_proj-660.jpg

:D Exactly it.

Here's a quick sketch of the design (painting not done):

http://img822.imageshack.us/img822/5157/butanecannon.jpg


I ran some tests on The HGDT and it showed that with one grill igniter ...

No No!! LOL I would never hit someone! I was just using something for a reference to the power. I could have used newtons, but that means I need to get my calculator.

all I am really asking, is will it come out of the barrel at a reasonable speed?

I will use that sim, thanks!

I saw that the D9 weapons have a nice canister on them similar to a small propane tank, and I designed a cannon with an automatic fuel meter and "push forward" reloader.

I am extremely happy with everything so far and I will post designs and pics of everything.

My question to the experienced ...

Thanks! I think that making your piston light but strong should be pretty good as it gets out of the way quickly and does not bash out the back of your chamber so easily. Is you make the piston, you don't need to seal the outside, just put a piece of rubber the diameter of the piston on the back of ...

I saw the GGDT program, but I first have to install the python software, I shot the grape with the barrel horizontal from one meter, then i worked out the speed by how far it went before hitting the ground (with the time it takes to fall 1m and the distance it traveled) so I didn't take in air ...

Yea, its just a ball valve, I made another valve, but I found pulling the ball valve open fast was plenty good enough. i was just wondering how worried I should be about having a spring instead of a stopper. The piston is made out of pvc pipe with a foam core.

I know, (ashamed) I am not proud of ...

My first cannon was just a ball valve water balloon launcher that wasn't very efficient.
However, I'm really happy with this thing, as you can screw on the barrel you want, and the tank you want.
I made a pretty nice piston valve with a strong spring I got from a sprinkler. I wanted to put a bumper ...