Drive a load with external power source using NPN and Arduino output pin

Using Arduino Project Guidance
#1

Here is an image http://i.imgur.com/sCGoZ.png of the components. It should be very simple. I have a Nerf gun, 12v power source, NPN transistor and Arduino.

I want to fire the gun (connect to 12v battery pack) when pin 13 goes HIGH via NPN.

I have tried a few configurations but nothing works, either I get nothing or the transistor gets so hot it starts to smoke.

I have been using both 2N3904 and MPS2222A which I can get working with LED fine so I know they work.

#2

You need a series resistor between the base of the transistor and the Arduino pin, like this:

#3