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Cut a 121in (1.32.5cm) section from a wire hanger if you want a dart-like projectile. Otherwise, you can use a BB or ball bearing as your projectile. Feed the projectile into the barrel of your coil gun. You can use any projectile as long as it's magnetic. Place the battery back in its terminal so a current can run through your circuit. Locate the triangular metal piece on the back of the circuit and press down on it with an insulated screwdriver. Hold the screwdriver there for 5 seconds so the capacitor can fully charge. Don't touch the capacitor after it's charged or else you will get shocked. Once the capacitor has been charged, flip the switch into the on position to shoot your gun. Turning on the switch completes the circuit and charges the coils around the barrel, propelling the projectile forward. Once you shoot the gun, leave the switch on for 1 minute so any leftover voltage in the capacitor can discharge safely. Make sure your gun isn't aimed at any living thing since it will shoot quickly.
Load a metal BB or piece of wire hanger into the pen barrel. Put the battery back in the camera and charge the capacitor for 5 seconds. Turn on the switch to shoot the coil gun.