Carriage ballista- how were they used?

[fretensis10]

Any thoughts on how the Romans used carriage ballista (carroballista)?

It seems a rough ride to sit in the back with one of those things and I wonder if it soldiers would actually try to load and fire one of those things in a wobbly carriage. Perhaps they used the carriage for rapid movement and unloaded for a stable firing platform and quick getaway?

[D B Campbell]

Unfortunately, it isn't known how the carroballista functioned. In fact, it is mentioned only by the late writer Vegetius, who says that each centuria had one, along with mules to pull it.
In battle, he says, they were positioned in the rear, to shoot over the legionaries' heads. The implication is that they were mounted permanently on carts, and manoeuvred into position.
By chance, Trajan's Column (scene XL, casts 104-5) preserves an illustration of two machines which are probably carroballistae. Each is drawn by a pair of mules. The machines appear to be in action, with one legionary loading the slider while another operates the windlass. If we are to take the illustration literally, then the machines were shot over the heads of the mules.
Whether or not this is actually feasible has never (to my knowledge) been tested. I wouldn't have thought that a two-wheeled cart presented a very stable base, particularly when harnessed to draught animals. But the machine was perhaps not designed for precision shooting, but only for laying down a barrage (and mules are notoriously stolid, after all).
I think some experimental archaeology is called for ..!

[Titus Martius]

I think, that they shoot from carts on Trajan's column.