Take a set of rings, and wrap wire around them to create electromagnets (like you might have done wrapping wire around a nail in science class). Make sure they're all wrapped in the same direction, so that the north-south poles line up. Set up some very high-charge capacitors and switch circuitry in such a way that you can power up all the rings at the same time. Now put a chunk of metal at one end, behind the "first" ring, and throw the switch. The magnetic field will pull the metal toward the center of the ring. Since it's open, the chunk will fly right through it and on to the next ring, which will also attract it in turn. And so on, and so on. If you've done it right, with precision guide rails to keep everything centered, the metal chunk will go flying out the far end like a bullet.