Hey cool!
Our team is debating whether to build one as well.
A simple robot mounted cannon:
http://www.chiefdelphi.com/forums/sh...t-shirt+cannon
However you could also reach for the sky with a system like team 254's Shockwave robot. Lots of info here:
http://team254.com/shockwave/
Thread discussing team 254's Shockwave robot:
http://www.chiefdelphi.com/forums/sh...t-shirt+cannon
And your can always try searching CD for more threads, or searching the net for a (non-FIRST) example. However, remember that some examples aren't
guaranteedto be safe. You would really want to carefully design any system that would be used for the general public.
Hope this helps,
Daniel D'Souza