This is a 3d printable version of OGG Craft's rubber band gun 

To assemble: glue the barrel onto the left chasse making sure the notched end is facing away and the holes line up

Place the trigger and releaser as shown in the video

The right side should friction fit, but if it doesn't then use glue for that as well

Wrap a rubber band around the front of the trigger as shown in the video

You can also roughly smear hot glue onto the release to increase friction and make sure the mechanism works well

YouTube link for original design: