Thanks.I just built a spud gun w/ my Uncle, and we test fired it a fewrounds. we'll post pics and maybe video soon.

I have noticed in several pictures that some cannons have computer case fans in them. What is the purpose of this?

I believe it's loaded with about 500 lead balls for taking out lots of ducks or other birds. Kind of ruins the point doesn't it?
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Just to expand a bit, THe punt guns of old would use a one pound keg of powder for propellant and about 5/8 to3/4 pound of shot. That's one HUGE boom.