This frame can serve as the backbone for almost crossbow and can handle several different arm styles, "barrels", and trigger mechanisms. Feel free to use the designs I have or create your own (please post instructions for those as well).
Step 1: Make the Front of the Crossbow
The front of the crossbow is designed to give many points to attach the arms of the bow to, allowing you some freedom when designing arms for it.
NOTE: The 2 connectors are light gray (2 connectors).
Step 2: Making the Body Sections
Feel free to make the crossbow as long as you wish. Make sure that the last body section uses the dark blue connectors.
Step 3: Reinforcing the Frame
Step 1: Make as many of these triangles as you have body sections
Step 2: Just a reminder that you use orange connectors in every triangle except the ones at the front and back (You should have red connectors on top in each one--my mistake while taking pictures)
Step 3: Again, make as many of these as you have body sections
Step 4: Attach the reinforcing triangles with yellow and red rods (or whatever works for the length you chose)
Step 5: Connect the white circles with yellow rods, going through each triangle. Note the tan clip at the very front of the bow
Step 4: Some More Reinforcements
Step 1: Make this rail
Step 2: Attach the rail to the triangles, and put the end with the tan clip at the back of the frame
Step 3: Just another picture to help you see where the rail goes
Step 5: Frame Complete!
At this point, the crossbow frame is complete. It is up to you to design and build arms, barrels and triggers that you think are effective. Please post links to these instructions in your comments. In about a week or so I will post some of the components I have built.