Years ago I found this image of a 4th century AD Roman crossbow, taken from a bas-relief in France and reproduced in a 19th century text, Dictionnaire des antiquites grecques et romaines: Arcuballista, Manuballista (according to wikipedia). I was drawn to making it because it was such an odd little beast. Unlike later crossbows, there is no shoulder stock or stirrup. It appears to have a rolling nut and a groove for the bolt. (It also looks like the string passes inside the stock, but I'm going to assume that was artistic license on the part of the stone carver.)
I made the stock out of oak, then put the project aside for a few years because I didn't feel up to tackling the prod (bow). So unfortunately I do not have any photos of the early stages of construction, but it's pretty straightforward. I did the inner shaping first (just a simple slot for the trigger), then sandwiched the two halves together. I turned it on a lathe to create the handle, and that's about all it took to make the stock, as I recall.
I would like to mention at this point that I made the thing way too small. As such, this is a very, very low power crossbow. It is still fun for paper targets though. Don't worry, no GoPros were harmed in the making of this video.
Also please do note crossbows are dangerous weapons and may not be legal where you are.
Step 1: Roller Nut and Channel
In the photos you can see the channel for the cylindrical nut. After the stock had been shaped, I clamped it down to the drill press and cut out the channel with a 1 1/4" Forstner bit, the same diameter as the nut. I made the nut from oak dowel of the same diameter, then shaped it carefully with a bandsaw to make the prongs that hold the string. I also inserted a short length of all-thread through the nut from top to bottom to form the 'face' that the tickler (trigger) rests against.
Step 2: Tickler (Trigger) Mechanism
I placed a little section of spring in the slot below the roller nut channel to return the trigger mechanism.
The tickler was made out of 1/4" steel bar stock, done at a local welder's. Given the low power of the bow, brass would have been fine. You can see how the tickler is held in place with a nail that it pivots on. The idea is that the end of the tickler, held up by the spring, rests against the all-thread face of the nut, preventing it from rotating until the trigger is pulled.
You can also find a scalable vector graphic of this idea on wikipedia.
Step 3: Prod (Bow)
I was finally motivated to finish the crossbow when I learned you can make functional bows with some PVC plumbing pipe, a heat gun and a couple of clamps. PVC is easily flattened and shaped when hot. I will not go into detail here because there are already quite a few Instructables on the topic and lots of resources out there (and I am not an expert). I highly recommend Nick Tomihama's "Backyard Bowyer" books for anyone who wants to learn more.
The prod sits in a slot in the front of the stock, at a slight angle to prevent string friction. I bound it in place with polycord which I also used to make the string. It's not ideal because it is too stretchy but I just wanted to see if it worked.
It did! Thank you for reading.