You get a rule. You get 4 square magnets. You get 9 steel balls. You get some tape, to tape the magnets to the rule evenly.
Put 2 steel ball each in front of the rule as if they were sticking to each other.
Put the last steel ball at the behind of the rest of the steel balls, for example:
0 = Steel Ball.  = Magnet. ~ = Rule.
Steel ball that fires. ---> 00 00 00 00 0 <--- Last steel ball.
Push the last steel ball into the magnet and watch the steel ball that fires move away from the rule at a very high speed.