This is the design we are planning on using:
http://www.youtube.com/watch?v=l8U9u2iqMEs
The plan is to use two 550 motors to spin the four 12" wheels we are machining from aluminum. The wheels have beveled edges to provide a lot more surface area with the balls when shooting. We are aligning them vertically so we can control backspin and minimize drift from side to side. We will be using a pair of PID control loops to get the RPM on the wheels as accurate as possible. The RPM itself will be tracked via hall effect sensor and magnets.
The students have done a lot of math and prototyping and everything seems to work out as planned. I'm actually very hopeful for an accurate long-range shooter to be honest.
-Mike