Don't laugh

I hope the ATF isnt reading this, I need a little help. You might think I'm crazy but I recently built a cannon that shoots tennis balls filled with sand. It can shoot about a mile, and wake somebody up from about 3 miles away. I am safe about using it, but as with all fun things I'm sure it is very illegal. Is there anything I can do to make this legal? Can I register it? I've heard stuff about how muzzleloaders are in a different category than other guns, is this true? Any help would be appreciated.

I have seen similar cannons at black powder shoots, get ahold of someone who does this stuff, I am sure they can help you.