
When it comes to internals it's a basic combustion. Here some specs:
Chamber: water bottle
Style: coaxial
Barrel: 1/2 inch Thai pvc (I live in thailand)
Ignitor: grill lighter piezo
Main propellant: spray on deodorant
Some Pics:

In the Chamber:

Layout:

The gun has a barrel knife and is breech loaded.
The piezo:

The piezo just broke and Ill be redoing the whole backside when I get a new one
The Video: enjoy

[youtube][/youtube]
You may notice that I close the top of the bottle when I fire it. This is so the pressure builds up inside before firing and it creates a loud BANG!
Hey I just unghettofied the entire backside of the gun after leaving the thing in a closet for 4 months. Here it is:)
From this:

To this:

Me pressing the piezo now concealed neatly within the gun:

tutorials up!