POTATO LAUNCHER
SPUD GUN
TATER CANNON
SPUDZOOKA
Call it what you will, but the result is the same: forgetin' AWESOME

Disclaimer: you MUST check your local state and city legislation before even bothering with building a potato launcher. Some states have outright outlawed spud guns (California), whereas some states recognize them as firearms (parts of Florida). If you want to read more, go HERE, but keep in mind that it may not be legal to build, own or fire potato launchers in your area. They should be treated as firearms as well: finger off the trigger until ready to fire, don't point at anything you don't want to hurt, and keep it unloaded until ready to fire.
A lot of people know what a spud gun is already, but JUST in case you don't know,
you should probably crawl out from under the rock you're living under here is the definition by wikipedia:
A spud gun (also known as a potato cannon or a spudzooka) is a pipe-based cannon which uses air pressure (pneumatic), or combustion of a gaseous fuel, to launch projectiles at high speeds. They are built to fire chunks of potato, as a hobby, or to fire other sorts of projectiles, for practical use. The projectile can be dangerous and result in life-threatening injuries, including cranial fractures, if a person is hit.
THE PROJECT
My dad and I MIGHT build a potato launcher modified to launch tennis balls for our dog. We have a couple ideas in mind, and I've been working on putting together information to build it, as well as talking to people who have experience with potato launchers in general. I've created this topic both to learn more about potato launchers, and maybe inspire some members of the BL community to either post their already made spudzookas or create their own. If I do go through with the creation of a potato cannon, I'll see about posting pictures of it. If you have knowledge of pneumatics, potato cannons or air compression powered weaponry, please share it here (for science), and maybe learn something from someone else as well.
If you you're interested in building a potato launcher and want a good place to start, take a look through these links:
How to build a spud gunElectric powered spud gunLaunch supersonic spuds with a PVC cannon! <-- recommended
25mm pneumatic sniper rifleFormufit.com (website specializing in PVC piping)
AdinX's to-do list & planned features:mounted tennis ball launcher, maximum range of 100ft, below 15lbs, air compression + simple firing mechanism
• complete blueprints for entire project
• purchase materials for entire project (projected cost: $80)
• Assemble quadropod mount for launcher (solid PVC structure, weights in bottom of legs, estimated height: 3')
• Assemble double rotary joints for launcher mount (up-down/left-right)
• Assemble air chamber (preferably max of 300 PSI capacity)
• Assemble release mechanism (probably going to use a sprinkler valve like in the 3rd link)
• Assemble primary barrel
• connect air chamber, release mechanism and barrel
• test fire at 50PSI
• test fire at 100PSI
• test fire at maximum capacity
• go to local park, attach cannon to tripod, take dog off leash, fire at will