Are air cannons legal in California?

I'm wondering if air cannons (potato cannons powered by air) are legal in California. And if they are, then what would happen if you accidentally hit (but not break) a neighbors window?

I decided to delete the previous thing I wrote, because it decided it was a not a "nice" reply, and the rules say to be nice.

Obviously you are worried about the legality of this thing you built:

That instructable looks pretty impressive to me, and my humble advice is to think less in terms of strict legality, and more in terms of personal responsibility

That is to say you should make every effort to avoid breaking other people's windows with your toy, and that's really the best way to stay out of trouble.

My guess is that if you don't injure anyone, or break anyone else's property, then the law will probably leave you alone.  Seriously.  The cops have more serious matters they should be worrying about.