I found this pen at school the other day and was taking it apart (like I do all pens) and I realized that it would make a really nifty ammo/barrel. So I was looking at it all day thinking of what to do. That night I went to the hardware store and picked up parts for my design.
Here's what I ended up with, a single pump, self contained mini coaxial in a tee:
The wood on the pump is to keep the pump centered.
How it works:
The pump is pushed down to compress the air, when the end of the pump passes the hole for the hose, the air behind the piston is vented and POP!
The tee is 1/2" and the pump housing is 1"
I thought I was just going to use it for plinking around the house, but it ended up being more powerful than I thought. It put a small hole in the wall at 20 feet away, and bent the tip of the ammo when hitting the plastic around my window at 15 feet(also left a dimple in the window plastic)
I found that I could also use the core plastic peice from a bic mechanical pencil with a toothpick stuck in the rear as pretty good ammo.
The piston is made of a rubber stopper, and I used a homemade spring (paper clip) to reseat the piston.
Total it cost about $20. My trip to Ace to get all the parts cost $16, but I already had the pipe, the o-ring, and the pen.
Hehe, this thing is really fun, I'm off to play with it more