Chi,
It is really pretty simple, we cheated by pulling an existing chassis out of the stack of old robots piled in the corner. It is 6 wheel drive with the center wheels 1/8" lower then the others, thus the ability to spin like it does. The entire drive train was in place, we did have to replace some gears which had been cannibalized. We used the old first controller since we can leave it in place permanently and Victors for the speed control.
The launchers are 2" diameter and 5' long. We mounted them at 45deg to maximize range and to avoid the possibility of a flat trajectory shot into the audience. I want the shirts floating down from above, not being fired down someones throat.
We have 2 compressors each charging 2 3" x 30" chambers made of schedule 80 PVC rated at 260psi. The chambers are connected to the barrels through a 1" irrigation valve which is controlled by a SPIKE and the controller.
One of the barrels is behind a check valve so that they can be charged simultaneously but fired separately.
http://jalopnik.com/5646965/t+shirt+shooting-a10-warthog-monster-truck-is-everything-right-with-america" is the laucher that take the cake the pie and everything inbetween!