Rubber band shooting crossbow with trigger mechanism inspired by authentic medieval crossbow mechanisms. Shoots rubber bands with power and accuracy! The support arms you see pictured are fiberglass driveway markers like you can find in most hardware stores. The poles I used are 1/4"-5/16" (7.5mm-8.1mm), and there is usually a pretty wide margin on how thick the poles are, so you may need to pre-measure and adjust the mounting holes as necessary.

Make sure you print the pins at 100% infill, especially the trigger pin, otherwise they will likely break when you assemble the crossbow. 

print the main fire control group (fcg.stl) upside down, so you will not need supports.
the trigger should be printed on its side (the orientation where none of the hole is overhanging).

Everything else can be printed as-is. No supports are needed

Materials Needed

• Parts fresh off the printer
• poles for support (1/4" fiberglass driveway markers are perfect) 
• rubber bands 
• round files or sandpaper 
• yardstick (optional)

Fire Control Group assembly

Parts needed:

• fcg.stl
• triggerPin.stl and nutPin.stl (OR pins.stl)
• nut.stl
• trigger.stl
• round files or sandpaper

Orient the fire control group body so that you are looking through the main pin hole, the pole mounting brackets are on the bottom, and the smaller trigger hole is on your right.

In this orientation, the right is towards the rear of the crossbow, and therefore the left is towards the front.

insert the nut into the fire control group body so that the smaller D shape is pointing to the right and the nut hole lines up with the hole in the body.

attempt to inset the nut pin through the lined up holes, the pin should take quite a bit of force to insert and it should be very snug. If there is really not enough clearance, then file or sand the hole in the body of the fire control group (there is more clearance on the nut hole, so no need to file that).

with the main pin inserted, the nut should be able to spin freely inside of the body.

get the trigger and trigger pin. The trigger is assembled in a similar manner to the nut. The bulge on the trigger should be on the right so that it hangs freely, and the left end of the trigger acts as a catch which should stop the nut from spinning counter-clockwise when the trigger is rotated so that it is even with the body. The trigger pin is very small and is easy to break, especially if you did not print the pins at 100% infill. Again, the fit should be snug.

Crossbow Assembly

Parts needed:

• frontSight.stl
• rearSupport.stl
• assembled fire control group
• support poles
• rubber bands

Find out how long you want the crossbow to be. For best results, get several of the rubber bands you plan to use and stretch them to their limit on a yard stick and take note of how far you stretched them. Use this measurement as a guide for how far back you will set the fire control group from the front of the crossbow.

simply cut two even lengths of your support pole and mount all of the parts onto the poles. You can't mess up mounting the front or rear supports since the mounting holes are closed on one end. The front of the fire control group is the side where the mounting holes are flush with the edge of the body.

After mounting all of the parts on the support poles, position the fire control group where you want it to be based on the rubber bands you will be using, and take one rubber band. You will notice a notch on the top and bottom of the fire control group; this notch will hold a rubber band which will make the trigger springy and reset automatically. wrap the rubber band around that notch however many times it takes to make it tight around the trigger.

This single rubber band should mostly hold the fire control group in place, but if it still moves around too much you can glue the fire control group to the poles or tightly wrap rubber bands around the poles just in front of it so it won't move.

The supports can be secured by either gluing or rubber lashing, but really this is not necessary.

The crossbow is now fully assembled! (use the pictures to verify the orientation of the nut and the fire control group).

Firing the crossbow

The crossbow is loaded mostly like a normal rubber band gun.

First, set the trigger by rotating the nut counter-clockwise until it cannot rotate any more. the bottom of the trigger should now be flush(ish) with the bottom of the fire control group.

place the rubber band around the front post, then stretch the rubber band all the way back and hook it onto the part of the nut that is sticking out above the body of the fire control group. Be careful not to impale the rubber band on the pointy part of the front sight post, as that will likely break the rubber band when you stretch it and it will snap your hand (ask me how I know).

Aim the crossbow in a safe direction, and depress the trigger. Once the trigger pull breaks, the nut will spin freely, releasing the rubber band.