I would refer to:
http://www.chiefdelphi.com/media/photos/23941
and
http://www.chiefdelphi.com/media/photos/23942
These are good pictures of shooters. It looks like the 8 tooth gear to 84 tooth gear to another pair of 8 and 84 tooth gear. For the wieght of the balls it should be fine, but if you want to shoot heavier balls you may need more powerful motors.
Any questions PM me
~gdo