Master_W
A rail gun uses electromagnetism to fire it's projectiles at extreme speeds along rails in the barrel. It uses massive amounts of energy, and to make a successfull rail gun will need a lot of planning, I can't think how it can be easy. Also, making your own gun is illegal in, well, almost everywhere.

Bert K
Here's how to do it, complete with theory: As you see, it is simple enough in concept, and the size of the power source and components depends on what you want to do. In simplest form: two straight wires stapled to a board, a pair of metal wheels mounted on an axle (from a toy car, or the complete car itself) resting on them, a couple of pillows on either end of the board, a battery with one terminal connected to one wire and the other terminal to a wire which you can tap on the other wire, and there you are. .