Finding how much fuel/air expands during combustion?

[loco]

I've been asked to build a potato canon-style device that shoots either tennis or golf balls (probably golf balls).
In order to make some calculations for combustion chamber volume and barrel length along with fuelling options, I'm trying to find out how much an air/fuel mixture expands when burnt properly, and what ratios of air fuels are most effective.

I'm sure it's possible to have a barrel that is too long, so I'm trying to work out what optimal lengths and volumes of chamber are.

I'm thinking of using regular petrol ('gas') to power it intially. Take a look at my laughable 30-second sketch to get a really rough idea of how it's going to fit together.

Thanks in advance.

[pezman]

The best spud-gun site I've seen so far:
http://www.burntlatke.com/

[Mark]

A good potato gun should refresh itself after firing a shot. It gets boring very quickly having to air it out after every shot. Of course, pneumatic potato guns get around this problem, but that is cheating.
Imagine a real semi-automatic self loading "potato-of-some-sort-projectile" machine gun. Seems there ought to be a way to have a strong one-way flap valve that would admit fresh air and fuel after the shot using the vacuum phase to refresh the chamber, just as a pulsejet does. Then, you could have a clip of ammo and a fuel/air chamber that you could spark and fire every time you pulled the trigger.
Now I know this isn't an impossible design, I just haven't seen it without using tanks of air and/or pressurized fuel.
This would be something to strive for. And a lot more fun. Be the first on your block.
There are some nail guns that have a little fan and fuel, so you can see they have entered that realm, they just haven't gotten around to doing away with the battery powered fan all contained/secreted in the handle. An automobile engine and a pulsejet do it, but we want to slow it down a bit is all. Then we will have to ban potato guns. Oh, the irony. Nobody is going to welcome machine guns, unless they are firing multi-colored mini marshmallows.
Another way would be to drive a parallel tube with piston that compresses and stores a chamber of fresh air for the next shot.

http://www.hometownfavorites.com/images ... fca470.jpg
http://www.x-entertainment.com/articles/0796/6.jpg
http://images.google.com/images?svnum=1 ... mallow+gun

[metiz]

Just wondering why you would need any calculations etc. It's not a pulse-jet - it wil work regardless :P the bigger the CC the further the shot. after you make on, use trial and error to see how much gas It'll take

the big one is 270km /h at the very least - 30fps camera could not track the projectile traveling 3 meters