Pneumatic cannon are surely outside the scope of this forum. But you are still in luck. I once made a foray into these waters (and surely will again, good clean fun). There is a TON of info out there to be had. Google "spud gun" and "sprinkler valve spud (or potato) gun (or cannon)" You will be inundated with ideas. Emphasis on the sprinkler valve. I won't go into it here, but these are trick. $37 at Lowe's and you can put a veggie (or some fruits) through a plastic sliding board. (Don't confuse these things with toys). Good luck and stay safe, MIKE
PS: Don't even search YouTube, unless you enjoy seeing unsupervised teens doing really stupid stuff.