Dane, you start building these arround the size that can be carried and I might be interrested....If I can find the money! :-)
Just wonder how they compare to medieval crosbows? Better? The Same? Or oh my god why the heck did we invent rifles!!! 
Ah, but the smaller machine I am posting pictures of as I built it is man portable. It will come in around 15 lbs, and will be about 40" long in total. Not as light as a crossbow, but far more powerful, I suspect. Why would the Greek and Roman engineers spend centuries prefecting these weapons in the first place? The complexity of the rope torsion machines probably had the most to do with the almost total disapperance of these types of engines after Roman fell (a very broad statement, that!). I and some others are thinking of these as crew served, but one man can easily tote it around and fire it as quickly as you can cock it and get a new bolt in it. The rate of fire for this small machine must be really decent, as well, and I have read reports from Roman reenactors with large machines getting of 3 or 4 bolts a minute, so my assumption is that this machine has a high rate of fire when utilized optimally. I'm also guessing that this machine will be very accurate, to the point of using it for sniping at long range. Another advantage, perhaps, is that the arrow firing machines would have had a better range than even full powered war bows deployed by enemy forces.
I plan to make a leather sling for it so I can carry it in the field, in parades, etc. since it is relatively light.
I am seriously considering a power thickness planer to make future machines easier to build. The hand planing was one of the most time consuming processes thus far, as well as making the mortise and tenon joints. A power mortising machine would take care of that, but I would much rather have a thickness planer as the one major power tool for this work. I have access to other power tools, such as drill presses, so no worries there. A perfect historical way of doing this would be only hand tools from the Roman period, but that would be very challenging. No slaves to help out, either,
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What is so exciting and daunting about this project is that I am in unfamiliar or unknown territory all the way through, in fabricating each part, estimating volume of bronze, making patterns for all the metal parts (which I am starting this weekend), making rope, etc. I have ordered horsehair to start making the ropes for the spring skeins, which I am not worried about after making so many bow strings. The wife will love when I tell her I need a rope walk, lol.
I also get to take on (sort of,
) all these nifty Roman titles that Immunes, or speicalized craftsmen in the legions had. Ballistrarii (makers and operators of ballista), sagittarii (arrow makers, or bolts in this case), aerarii (bronze workers), and fabri, the men working fabricae, or workshops. Of course, each specialist would contribute to the whole machine, and the friends here who are contributing to this work do kind of fill that role, meaning my friend the smith from the Ozarks who is crafting bronze nails, and Loki, who is sending me that really special point forged by Hector Cole.
More pics soon.
Dane
