We evaluated a number of different piston configurations, some of extreme throwing distance and some that were too weak. All of them are completely pneumatic, including the one that's actually on our robot right now.
We ended up using a four small bore piston shooter mounted high atop our robot, as near to the overpass as possible. We used this lower-power-requirement/output shooter configuration because it was faster than a huge pneumatic shooter. With the fast grip/lift on the front and the relatively low air usage shooter on the back, we arrived at the fastest recharge time system we could think of.
Feel free to come by in atlanta, any of our guys would be more than happy to answer any questions... we're easily locateable by the highlighter yellow numbers and robot.
-q