I was given the challenge to make something that will shoot, and make it fully printable. I settled on making a printable crossbow. In the 3rd century to make a crossbow required a team of skilled artisans and weeks of time. I figured using 21st century technology I should be able to do about the same in about a day.
The bow design was basically designed by eye by what looked good. I choose the 2mm wall thickness for it after a few test prints of strips at different thicknesses. The 2mm wall thickeness gave me what looked to be the best flexibility and durability.
When printing this bow I set Slic3r to infill the material in a concentric pattern. What this did is made the flexible in the dimensions I needed it also helped with the trigger. The bow was printed with the handle side up and with support material. The part under the trigger needed support material.
The trigger design stumped me for a while. I was originally going to steal some design from google images. All of the designs I found had far too many moving parts. I wanted the bow to be simple and not have much assembly. After drawing every trigger out, I figured I can just use the plastic as a spring, which worked great.
You can download the model here : http://www.thingiverse.com/thing:60343