The exact form of the cheiroballistra handle depends in large part on the way the cheiroballistra was cocked. If we assume that one used stomach pressure alone, then a simplification of the Iriarte-style handle works very well. Here it's shown detached from the slider:
It's attached to slider like this:
It is locked to a strong pin attached to the case, and is partially auto-locking, i.e. a small tap with a palm is required to really lock it after it has climbed over the pin:
This type of handle works beautifully, but it limits potential energy storage of the cheiroballistra because hands can't be easily used to assist the pullback.
Cheiroballistra's closest analogy, the stomach-cocked gastraphetes, which used a normal bow as it's power source, had ratchets and pawls which served as a way to automatically lock the slider at certain intervals, so that the operator's hands remained free. Heron is somewhat unclear regarding how the gastraphetes was cocked:
After that they rested the end of the slider, that had been pushed out, against a wall or the ground and, holding in their hands the ends of the withdrawal-rest ΤΥΦΧΨ, they pressed the belly against the concavity ΧΨ. Straining hard with the whole body, they pushed the slider back and drew back the bowstring... (translation from Marsden 1971: 23)
If the above translation and interpretation is more or less correct, it would seem that both stomach pressure (push) and hands (pull) were used to cock the weapon. This makes perfect sense, as it allows use of a much stronger bow with no real downside. According to Marsden (1971: 46) Diels & Schramm (1918) described the cocking process well. Unfortunately I don't have this research paper yet, so I can't check what their exact interpretation was.
Regardless I think it is fairly safe to assume that hands were used to assist cheiroballistra's draw. Stomach pressure alone might - based on my current experience - yield ~100 joules of projectile energy at most for a 90 kilo person. Assisting the drawback with hands might add maybe 50 joules, depending on the technique.
So, how could we assist the drawback with both hands? Simply pulling the bowstring is out of the question because at the end of the draw, where assistance is needed the most due to the peak in the force-draw curve, the bowstring angle is too acute. So we really only have two viable options:
- Use hands to pull back the arms
- Use hands to pull back the slider
The first option is certainly doable, but the amount of hand movement is fairly limited and any performance improvement would probably be fairly modest. Care would also have to be taken to pull each arm equally, so that one arm is not rotated more than the other. This problem is essentially the same as with bows and crossbows - if otherwise perfectly balanced limbs are pulled unevenly, one limb will be strained more and will move quicker when the bowstring is released, resulting in erratic sideways movement of the bolt's butt-end. Due to a combination of high pressure, friction between the nut or the claw and (acute) bowstring angle the limbs or arms are unable to correct their position without assistance. In addition, this method of assisting the pullback would require the handle (χειρολάβη) to be able to automatically lock to the pin in the case, and P.H. does not describe any springs or anything similar that could serve that purpose. Unless, of course, we assume the pitarion (πιττάριον) is some sort of spring, which is of course possible, but not very likely in my opinion. Gravity-operated auto-locking only works if the slider is kept near horizontal, which is difficult at the end of the draw.
The second option - pulling back the slider with both hands - seems a lot more likely as it allows longer hand movement and requires no special technique. Now what part of the slider construction would one pull? This brings us back to the χειρολάβη, which translates literally to "something grasped with hand(s)", or "handle". For some reason other scholars have not been keen to interpret the "handle" as a handle (e.g. Wilkins 1995; Iriarte 2000). The primary reason is probably the size of the handle in the manuscript diagrams, where the handle is shown narrower than the top of the slider. As the proportions in most of the cheiroballistra manuscripts diagrams are generally way off, I don't see why we should think they're more of less correct when it comes to the triggering mechanism. In fact, when we disregard the proportions, the handle looks a lot like, well, a handle. Also, why would a part called the handle not be used as such, especially as it would actually serve two very useful purposes:
- Increase energy storage considerably
- Allow easy manual locking of the slider to the pin(s) and thus the case, doing away with the need for any auto-locking mechanisms