This is the time to enthusiastically compliment him on his great canon building skills and the very cool result. And casually ask about safety measures saying you don't want him ( it has to be a him ;) ) to get hurt. Either he will reassure you about safety, if not at least he sees you as an ally and won't aim in your direction..

I can't think of a better way to get rid of squirrels and gophers

I hope you called the cops before posting this. Holy crap my neighbor built a weapon that takes out ships is is something I'd be worried about.

Loved your description of the cannon XD But yeah, this is still worrying.

i would start recording everything, so in case something does happen, you have some evidence against him.

Well that's pretty impressive if he really built a cannon so well that it can be used on a "pirate ship". Unless he's blowing up trees near your house, which I highly doubt he's actually using live ammunition, you're being a bit of a jerk by calling him "psycho".

.....I'll trade you neighbors

I wish he was my neighbor. I'd love to shoot a cannon.

well call the cops...

More common than you may think, it is a historic collectable. Nothing to worry about at all, engage your neighbor in rich discussion, probably has a great story.