If you're doing a cannon this large, you have to do it as a combustion, because pneumatics are too slow to load at this scale.
Carefully meter in a crap load of propane (or hairspray if you have to).
To fire, go inside, shove yourself up the barrel, then press the sparker on the barbeque you no doubt have, using a long stick if needs be... If ignition fails, then crawl out of the barrel, wave your arms about and flail to mix up the gas.
Make sure you duct-tape up the windows on the shed, because they'll break, and you don't want any pressure leakage.
When you go inside, don't leave the door open too long or the gas will bleed out. However, after returning to the cannon, open the door after firing to carefully air out the shed for the next shot.
Also, wear a gas mask and fire suit - propane is poisonous in these concentrations, and you don't want to be poisoned by your cannon - and it would be nice to come away without third or fourth degree burns.
Place a trampoline, haystack, manure pile or swimming pool where you intend to land, because falling hurts.
A parachute is adviseable in case you miss your target area.
You are welcome to steal my idea, I haven't patented it (yet).
Novacastrian:How about use whatever the heck you can get your hands on? frankrede:Well then I guess it won't matter when you decide to drink bleach because your out of kool-aid.
...I'm sorry, but that made my year.
I was playing around with the thing found here >>>
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Had there been any kind of projectile the hole wouldn't be that round.
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