+ 1 for start ya bastard, oxy and acetylene also works very well for a few blast and if you use lpg make some kind of remote trigger because there is a good chance the bottom will explode.
As for ammo, frozen oranges go the best for distance, mangos are good, but usually only the seed comes out, for show we used to wrap petrol soaked rags with duct tape and dip them back in petrol.
Mat
there's a site somewhere with designs for a pneumatic (air pressure, not combustion) ones, you pump them up, then flick a switch which opens a solenoid valve. i guess that's technically legal because it's not combustion, but good luck convincing the cops of it.
Guessing illegal, simply because of the incredible power they have, they are basically a freaking cannon... Pretty sure that at close range they would be fatal/close to fatal, used to shoot well over 200m, or straight through both sides of 3 plastics buckets, car windows (Our own bush bashers, not other people cars before anyone flames me).
LPG, Hairspay combustible propellant ones are illegal. My oldest son made a few(of all sorts) with a mate and tested them on mates parents property and i was all for him doing it. He left a couple under my house when he left home a few years ago that I have cut up for plumbling Consider them very short barreled spud guns now. Meh, you can tell I really fear flying potatoes/oranges..
^^ You don't want be caught with sawn off spud guns man, they hang you by your toes for that. Never know when you might hide one under your shirt and go into a public place and off somebody.