The Gauss Rifle:
We need about four strong magnets. I suggest super strong cylindrical magnets (such as N35.500.500) with the same diameter as the steel balls. Cylindrical magnets can easily sit in the grove and can easily be secured in place.
We need some sticky tape. Again, almost any kind will do. Here we use Scotch brand transparent tape, but vinyl electrical tape works just as well.
We will also need nine steel balls, with a diameter that is a close match to the diameter of the magnets. We use 1/2 inch diameter nickel plated steel balls.
Start by taping the first magnet to the rail at the 2.5 inch mark. The distance is somewhat arbitrary -- we wanted to get all four magnets on a one foot rail. Feel free to experiment with the spacing later. If you are not using a ruler as the rail, then you can estimate the distances or measure them to ensure you are using the recommended distance.
It is very important that you keep the magnets from jumping together. They are made of a brittle sintered material that shatters like a ceramic. Tape the ruler to the table temporarily, so that it doesn't jump up to the next magnet as you tape the second magnet to the ruler.
Continue taping the magnets to the ruler, leaving 2.5 inches between the magnets.
When all four magnets are taped to the ruler, it is time to load the gauss rifle with the balls.
To the right of each magnet, place two steel balls. Arrange a target to the right of the device, so the ball does not roll down the street and get lost.
To fire the gauss rifle, set a steel ball in the groove to the left of the leftmost magnet. Let the ball go. If it is close enough to the magnet, it will start rolling by itself, and hit the magnet.
When the gauss rifle fires, it will happen too fast to see. The ball on the right will shoot away from the gun, and hit the target with considerable force. Our one foot long version is designed so the speed is not enough to hurt someone, and you can use your hand or foot as a target.
How does it do that?
When you release the first ball, it is attracted to the first magnet. It hits the magnet with a respectable amount of force, and a kinetic energy we will call "1 unit".
The kinetic energy of the ball is transferred to the magnet, and then to the ball that is touching it on the right, and then to the ball that is touching that one. This transfer of kinetic energy is familiar to billiards players -- when the cue ball hits another ball, the cue ball stops and the other ball speeds off.
The third ball is now moving with a kinetic energy of 1 unit. But it is moving towards the second magnet. It picks up speed as the second magnet pulls it closer. When it hits the second magnet, it is moving nearly twice as fast as the first ball.
The third ball hits the magnet, and the fifth ball starts to move with a kinetic energy of 2 units. It speeds up as it nears the third magnet, and hits with of 3 units of kinetic energy. This causes the seventh ball to speed off towards the last magnet. As it gets drawn to the last magnet, it speeds up to 4 units of kinetic energy.
The kinetic energy is now transferred to the last ball, which speeds off at 4 units, to hit the target.
Another way of looking at the mechanism
When the device is all set up and ready to be triggered, we can see that there are four balls that are touching their magnets. These balls are at what physicists call the "ground state". It takes energy to move them away from the magnets.