As with 488, we reached similar conclusions that using wheels to launch the ball would create too much wobble due to the compression of the ball. Our solution was also similar, using some sort of container or cup to launch the ball with a quick release of energy.
The angle of the rail is at 45 degrees. Due to the limitations of our workspace (ceiling height), we're only using about 50-70% available power/length of the rail, and of course it can be further adjusted by manually changing the tension. The video shows shots fired at 21'.
We were making about 3 in every 4 shots. We made all of our shots when the ball was placed at a certain orientation (FIRST logo face up), but it veered slightly when it wasn't placed face up.
http://youtu.be/3Y3a4f30xXY
Any questions/critiques/suggestions? We'd love to hear some possible improvements to the design.