POTATO Cannon!

This is a cool thing that I saw my neighbors use one. It is AMAZING and can be built for less than $25.00 (It's way worth the money). There are also some videos on Youtube that will help you understand how to build this if this isn't sufficient enough.

Get all your parts ready for assembly.
Cut all your pipes to the correct length using a saw.
Glue the end cap holder onto the end combustion chamber pipe (Fatter, shorter one) Make sure you don't get glue into the threads.
Take the 3" to 1.5" reducer and glue the 3" end to the other side of the combustion chamber.
Glue the barrel to the other end of the reducer.
Put two 3-inch nails into the 3" section on either side so there about 1 cm apart.
Put small alligator clips on the leads to your BBQ sparker.
Let it dry for a day, then you're ready to fire.

Ensure a tight seal around the projectile. This prevents the combustion gasses from escaping around the projectile instead of pushing it forward and will result in inefficient shots.
Cut each potato in half and put it on a board or hard ground. Use the barrel as a cookie-cutter to force a perfect circle of potato into the muzzle and force it down with a ramrod.

Open the end cap.
Spray propellant (almost all hairspray will do) inside the chamber and close end cap tightly. Seven seconds of spraying is plenty.
Connect the alligator clips to the nails in the chamber.
Aim away from all people and click the sparker (it may take a few sparks so don't be discouraged if it doesn't work).

Instructions Source: http://www.wikihow.com/Build-a-Potato-Launcher