I made several potato cannons over the years of the hairspray/rightguard type. Many plans on the internet
so I'll spare you the details of how to make them. Here is what I have learned: These things are dangerous as hell! Reenforce the combustion chamber and joints from and to it with duct tape, then a towel, then more duct tape. Pvc shrapnel is a bitch!!! Only use rightguard. Hairspray works as good, but gums up everything and smells nasty.. Starter fluid is much more volital and can and does blow the cannons up!!!!!! Never hit someone directly at a range of less than 50 yards.... Believe me! You can kill someone with these things.
Lastly: My favorite use for the cannons was for hyper-extended casts, while surf fishing
for shark. Have a partner hold a large open faced reel with the bale open. Cram a potato snugly down the tube. Carefully put the hooks, bait (squid for me), and weights on top of the potato; prime the cannon, hit the striker, and the joy of watching a couple squid shoot 250 yards at a hundred miles per hour, far out into the surf, is a joy that few people ever get to witness!!!! It brought tears of joy to my eyes!!!!