Summary

This uses a stepper motor to add an electric drive for the Gatling Rubber Band Gun (http://www.thingiverse.com/thing:647475). The crank arm from the original Gatling Rubber Band Gun should be removed, and this add-on secured to the Rear Body using zip ties.

Requires a bit of electronics and soldering. Details are in the instructions.

https://www.youtube.com/watch?v=FMEkEvbr3Qs

Instructions

3D parts designed in FreeCAD (fcstd file is included) and printed in PLA using Cura. Electronic parts laid out using DIY Layout Creator.

Print using the individual component's STL file, and refer to the image to see how everything fits together. For higher speed (...and less torque), use v2 of the gears.

Besides the 3D prints, you'll also need...

Stepper motor (I'm using the ASTROSYN Y129, http://sg.element14.com/astrosyn/y129/stepper-motor-1-8deg-12v/dp/9598642, but you can use any other NEMA stepper of the same size)
Atmel ATtiny13 Microcontroller (1 pc) (you can also use the ATtiny 85, but will need to make some minor changes during compilation and upload).
7805 Voltage Regulator (1 pc) (...or any 5V regulators will do)
BS170 MOSFET (4 pc) (you can use the 2N7000, but if you do, you'll need to mount it in reverse)
Terminal blocks (6-ways) (Something like this http://sg.element14.com/camdenboss/ctb3051-2bk/terminal-block-wire-to-brd-2pos/dp/3882615?ost=3882615)
On/Off switch (1 pc) (This sholud be fine http://sg.element14.com/multicomp/1ms4t1b1m1re/switch-spdt/dp/9473467)
Stripboard / PCB board
Battery Holder (8 batteries)
Wires
Zip tie / Cable tie

Compiling...

You can used the provided hex file for direct upload to your ATtiny, or you can compile using.
avr-gcc -g -Wall -O2 -mmcu=attiny13 -c -o stepper.o stepper.c
avr-gcc -g -Wall -O2 -mmcu=attiny13 -Wl,-Map,stepper.map -o stepper.elf stepper.o -lm
avr-objcopy -j .text -j .data -O ihex stepper.elf stepper.hex

Electric Drive Add-on for Gatling Rubber Band Gun