We had the same discussion a few years back and opted to go a different route. We made a giant nerf gun which shoots 1' sections of pool noodles. At top speed it will now shoot a "nerf bullet" about 70' which can then be traded in for a t-shirt or whatever you choose. It also allows for good demonstrations with younger kids and has little chance of injury. The pneumatics only feed each noodle into the spinning wheels which work like a baseball pitching machine.
here is a picture location
http://www.chiefdelphi.com/forums/sh...099#post776099