Regulations on cannons?

[mcholak]

I know this has absolutely nothing to do with CCW, but I'm watching the Mythbusters use a cannon right now and I'm wondering what it takes to buy/own one. I don't really want to buy a cannon.....well, that may not be entirely true....but I'm curious if you can legally own one without some type of major permitting process. Also, are there different federal and state laws on something like this?

Just a curiosity. Anyone know anything about this type of thing?