I know that this thread is dead, but I finally got around to revising this design to increase dart capacity and allow for what Ice Nine described above. I figured I'd share that progress.
http://i.imgur.com/U5mPO7E.jpg
http://i.imgur.com/ArMoeVP.jpg
I bought another "stomp rocket bladder" off Amazon ($3), which actually turns out to be a foot pump for exercise balls, and made this 2.0. The new version shoots up to 4 darts and uses a larger bladder than the original. The larger bladder allows you to step/run on it. I don't recommend that however, because it's an awkward maneuver that yields less power than just whacking it.
It still operates the same way in general however.
The barrels are 1/2" PVC, with 1/2" CPVC supports underneath. I used two old tagger heads as extra cushioning for the supports. I used JB Weld to lock the barrels into place.
Here's a size comparison to give you a rough idea.
http://i.imgur.com/vkgSUMQ.jpg
Ranges:
Shooting 1 elite out of each barrel = 60'
Shooting 2 elites out of each barrel = spread of darts between 30' & 40'
It's of great use during indoor wars/cqb and is a pretty straightforward project.