This demonstration of the Gauss gun effect uses a 660mm long section of acrylic tube to house rare earth magnets and ball bearings.
One end is raised slightly to allow the first ball to begin its movement ad there is a rubber stopper at the other end to prevent any dangerous projectiles.
The Gauss Gun provides a safe but dramatic demonstration of the acceleration of a body in a magnetic field and the conservation of momentum. It uses groups of high strength neodymium magnets to accelerate steel ball bearings with the whole assembly contained in a closed, transparent plastic tube for safety.
A great discussion experiment on the nature and strength of magnetic fields, momentum, conservation of energy etc.