I am prohibited from owning one (something about small children and the china cabinet), but not from posting photos and videos.
You can find mini-cannons online for $40-$50. They're not that tough to make. Some shoot BBs, other hurl a powerful .30 caliber cannonballs (a.k.a. 5/16" steel slingshot balls available at sporting good stores. They are constructed and work pretty much just like the Napoleonic-era cannons. You simply load readily available black powder like Pyrodex or Triple 7—or you can simply make your own powder from matchheads, tamp it with a pen, light a fireworks fuse, and you are ready for mini-battle.
Here's a mini-cannon in action:
Related links: Once a Spy