Saw one (more of a mortar then a cannon) a guy built that launched bowling balls (about 400 yards, I think). Yeah, yer right on track asking about legality before undertaking such a project, might help to specify which state you're planning on risking terrorist charges in
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When Banjos are outlawed, only Outlaws will have Banjos
The Bible is my lawbook. I turn the other cheek when applicable, and spend the rest of my days resisting evil at every front, until I have breathed my last breath.