This is my how-to I posted on instructables, and thought I would share it with you guys. If you follow the link at the bottom, it links to my instructable page where I teach you how to build one similar.
There's also a video on the how-to page of me shooting numerous pieces of furniture.
This beast is capable of shooting a golfball over 500yards.
Just check it out here for step-by-step instructions how how to make it:
Golf Ball Gun!!! How To Make One That Can Shoot 400+ yards! - Instructables