Manuballista

[D B Campbell]

Could the term have been synonymous with Scorpio?

Ah, you're thinking of Veg., Mil. 4.22.6, where he writes: scorpiones dicebant quas nunc manuballistas vocant, ideo sic nuncupati quod parvis subtilibusque spiculis inferant mortem ("what they now call manuballistas they used to call scorpions, naming them in this way because they inflict death with small, slender darts"). Whether or not manuballistas were once called scorpions, there is no reason to equate them with the stand-mounted arrow-shooter that I presume you're thinking of. The name "manuballista" (being cognate with Greek cheiroballistra) seems (to me, at any rate) a better clue as to the machine's design.