roman crossbows

[D B Campbell]

(small) roman crossbows. ... Any reference for their use in the army?

The most comprehensive publication is: D Baatz, "Die römische Jagdarmbrust", in: Archäologisches Korrespondenzblatt 21 (1991), 283-290.

The basic evidence are the two Gallo-Roman reliefs from Haute-Loire (Solignac sur Loire and Flur Saint-Marcel) which depict a crossbow amongst hunting equipment. I never managed to track down the actual reliefs, but you'll find sketches in Daremberg-Saglio, figs. 467 and 468, Maarten.

Edmund Saglio (1877) already equated these with Vegetius' arcuballista, on etymological grounds.

For hand-held mechanical weapons in the Roman army, see D B Campbell, "Auxiliary artillery revisited", Bonner Jahrbücher 186 (1986) 117-132.