As a kid we used to make them out of beer cans...Beer cans were made of steel back then, not thin aluminum. We used tennis balls, and would have several kids with baseball mits way down in a field, and launch the tennis balls for them to catch.
I could just see my two grandsons using that. Potatoes are a bit too expensive for projectiles though, have to find something to use that would be recoverable and re-useable like foam rubber balls. What would be really cool for adult usage would be some exploding grenades with colored smoke that the fuse is ignited by the cannon fire, just like the old cannon balls.
I did that too. Loads of fun, but I'm not sure how safe.
Camera mounted to the tube with a monitor on the hood would probbaly work better.You need a set of cross hairs mounted to the hood to aim it.