Extremely powerful I shot a home-made hot glue gun cartridge at my garage door at 100 psi, it dented it so much the paint on the other side of the door fell off, great fun.
How It Works:
it works by building up pressure in a PVC chamber using a bike pump this pressure is held back by a ball valve attached to the trigger mechanism, when the ball is opened the pressure will be forced down the barrel pushing (at high speeds) the projectile out of the barrel with it.
How The Trigger Mechanism Works:
The Trigger and spring mechanism. it works like this: the bolt is attached to a spring, when the bolt is pulled all the way back to the trigger block it locks on to it so the sear is holding the pressure of the spring, when the trigger block is pulled down by pulling the trigger it unlocks the bolt which goes shooting forwards pulling the ball valve lever (attached to the bolt by a spring) forward quickly opening the valve causing the gun to fire.
Oh yeah one more thing the photos are shown without the bolt or ball valve string this is to prevent the springs from becoming over stretched.
got the idea off this video on youtube - http://www.youtube.com/watch?v=pA8VSM9R4nY&feature=related