I've been playing with the idea of what cool things I might be able to do with soda bottle preforms for a little while now and this is what I've come up with. The dart gun in this video uses two different firing methods that are commonly used in larger potato cannons, a pneumatic firing option, as well as a simple combustion option. A coaxial piston valve is my personal favorite mechanism for these sort of projects, and this video shows how it works quite well through the clear chamber. For the combustion design the fuel of choice is isopropyl alcohol, preferably 70% or greater.
How I made the darts in this video: http://youtu.be/BRUYdmOKLyM
How to make a mini cannon from a lighter (another mini airsoft cannon): http://youtu.be/QGj8xMDRSQ0
How to make water bottle rockets: http://youtu.be/YRtFY_O0HcU