Our new cheiroballista...

[Nerva]

I'm thinking braided would be easier to make, but reverse twisted rope may perform better, at least using horsehair. That is based not on scientific proof, but my instinct. They may act the same way in a torsion spring...I just don't know.

I would have to agree with you. You can produce a denser rope with twisting. This means you can probably get a greater volume of material into the rope and hence store more energy. As with all substances though it boils down to the tensile properties of the material. I do not know the specific modulus of horsehair but if the sheer strength is superior to its tensile strength then braiding would work better. I cannot remember what why Chad Houseknecht constructed his rope on Weapon Masters.