The prototype of a mini ball launcher which will be part of a remote controlled turret for my kids. It is completely 3D printed, except for a cheap, ubiquitous electric motor.
Read http://dlacko.org/blog/2016/06/25/3d-printed-mini-ball-launcher/ for details.
It is designed to be used with an R140 sized electric motor. You can use a different size, it is an easy change to the openscad source file, but be advised that the RPM really matters. If the motor is too slow, the ball does not fly too far (that is not much fun), if it is too fast, the launcher may be too dangerous (that is not for kids) http://dlacko.org/blog/2016/06/25/3d-printed-mini-ball-launcher/ provides some hint on how to calculate the RPM for a given range.
Use short M3 screws to attach the rotor the the motor shaft and longer M3 ones to fasten the cover to the housing.
The balls should be ~15mm plastic ones.
It is just a prototype, you still need some electronics to control the motor for the proper speed. Use it at your own risk, with a strong motor it might be harmful.