With a huge value cap then the coil will still be energized when the projectile is trying to leave the coil. Therefore the coil would try to pull it back, putting on the brakes big-time.
The most effective way to control the coil would use a photo-detector to turn off the coil when the projectile reaches its center. Then all the power of the coil would be used to accellerate the projectile.
If the projectile is magnetized, the polarity of the coil could be switched to pull it in then push it out, doubling the thrills. :lol: