Bad ass desolate. Looking really keen. What are your materials? Aluminum alloy and some sort of HDPE?
Push comes to shove you may be able to modify something like a cordless drill (cheap one, mind ya). Not sure what all electrical motors are available from hobby shops or professional sources, the only ones that come to mind are stuff like RC boats or some research automotive stuff.
I suggest an airsoft gun motor they have quite a bit of power and torque and all you need to power it is an RC car battery or if you really want to go the distance get a custom airsoft battery made
If you have the motor, and the barrels turn, it might as well fire. But forget it. In the game, there is an ammo box and a ammo belt attached to the gun. What do you plan on using for those two parts desolat? Real bullets perhaps!?