This is a fully working crossbow designed to shoot toothpicks !  
It is remarkably powerful for something this size, it will penetrate cardboard and has a maximum range of 15 meters. 

This is a modular design that allows you to fit which ever Prod (bow) you like 
they simply slot in and you use a small section of filament or toothpick as a retaining pin. 

3 Part trigger action 
The trigger mechanism is a remix of https://www.thingiverse.com/thing:5153589 
which is a traditional 3 part trigger mechanism 
This gives a lighter and more reliable trigger pull 
as well as being an interesting mechanism to look at. 
The holes are 1mm intended for sections of 0.8mm paperclip steel. 
Each part will need careful deburring and trimming to get a smooth low friction operation (especially the nut). 

3 types of crossbow Prod 
take your pick Decurve, recurve, Elastic 

Decurve
This is the D shaped bow, it is modelled on a traditional steel bow and suits PLA's properties, 
It is a stiff and heavy movement but the design means that the material doesn't actually have to flex that much to produce allot of force. 
This is the more powerful and reliable design. 

Recurve 
Taken from a wooden recurve bow shape, this design is far more flexible and springy. 
The design is more efficient and should be more suitable for light weight bolts. 
But PLA has its limits and the lifespan will struggle 

Elastic 
designed to turn this into a sling bow, use small rubber bands joined by thread to make a rubber powered version 
 

Ammunition 
designed for 2x50mm toothpicks, cut the point off one end and make some flights from masking tape. 
If your really keen carve a convex point on them and dip into superglue for better penetration. 

Stringing 
the best way to string it is to use sowing thread and wrap it round 4-6 times before tying it off, so that its one big loop of thread coiled a couple of times. 
Then take off one end of the bow and twist it; which will make it shorter so the bows a littel pre tensioned and pull the threads into a single thicker cord. 
The string shouldn't be too thick or too thin, a little experementing will have it shooting reliably. 

Print settings 
i have provided the 3MF file which has all the recommended settings pre loaded,  i have adjusted all of the individual settings on each part 
most parts have adhesion helpers as these fine parts have a habit of warping or becoming detached. so Brims shouldn't be needed but you may find it more forgiving to use them. 
The action parts i normally print on a tiny raft their accuracy is important to getting it to work properly. 
once printed all moving parts are likely to need deburring or trimming with a scalpel to get it all running nicely.