This ball launcher uses the principles of magnetism and conservation of momentum to accelerate ball bearings along the rails and eject the final ball with substantially increased velocity.
Video and all my other work can be found on my blog
Connect the two rails together using the spacers.
Slide two disks up the rails and leave them near the magnet indents.
Place 2 magnets in the indents and then slide the disks down over the magnets to hold them in place.
Use 3x 12mm magnets and 10mm ball bearings (catapult shot)
No glue is required.