Glue is also optional, but your milage may vary with the tolerances of your printer.
I loved the simplicity of the original rubber band gun by @gavilan, but the need for screws, bolts washers, and no idea what size elastic band to use was a bit of a downer. So I built this with the innards.
I also made the barrel shorter to work with size 10 rubber bands, well a standard is as good as a rest. The trigger elastic band is a size 16.
Bring on the zombie apocalypse!!