Legionaries with slings

[M. Demetrius]

Were these dedicated auxiliary slingers or legionairs using a sling

Not sure. Maybe somebody with a ready copy of Civil Wars could look it up. Mine is loaned out.

As a somewhat interesting aside, the sling bullets must have been able to damage or penetrate the helmets, or they wouldn't have bothered to build all those exo-armor baskets, eh?