The Notitia Dignitatum reported at least seven units of ballistarii (from ballista= catapult) but they are considered pseudocomitatenses unit. It is possible that they are deployed as border forces.
Below some of my Ballistarii. A mix of Gripping Beast (plastic and not), A&A and Warlord miniatures
The purpose of an independent artillery unit was to permit heavy concentration of firepower in particular during the sieges.
The ballistae described by Ammianus seems similar to the cheiroballistra sculpted in the Trajan's column.
The manuballista is a Gripping beast miniature. It is no possible to find this kind of war machine intact but it was re-created based on some parts found in Romania (the Orosova fortress)
|(Photo Baatz 1978)|
|(copyright Legio XV Apollinaris)|