We actually just switched to a pneumatic launcher. Getting the geometry of the launcher right is critical.
Here are some
photos and video of our launcher.
We found that moving the pivot on the launcher arm and moving the bottom pivot we could dramatically change the trajectory and force of the launch.
You should try to use more of the stroke. It will allow you to get more acceleration time through the launch.
We also adjust the hard stop at the end of the launch to change the trajectory of the shot.