Lego Shotgun!

A Pump Action Shotgun, with somewhat better range and power. Loop two rubber bands together to compensate for the longer firing pin. The pic below is dark, but you can sort of make out the rubber bands for the firing pin. This is the completed model.

For the first thirteen steps, look here.

Step 24: Take the chassis from step 13 and attach to the appropriate part under the shotgun. The grip and assembly to the gun can be completed as normal, without any other modifications.