Slingers were a separate set of ranged troops for the roman armies.

I can't recall precisely where I read it but what made slingers so deadly was their capacity to aim with precision on very small targets, unlike archers that would generally send arrow volleys in almost a suppression fire mode. A slinger could aim for the head, arms, eyes or joints for cripling or even deadly hits.

Slingers often used lead to create "bullets" by just smelting it over a camp fire, making holes on a patch of sand with the tip of a finger, and pouring the molten metal in. A volley of these small, extremely dense but compact and deadly projectiles would wreck havoc on enemy lines or could be used to target commanders to break the chain of command and demoralize troops.

There's a similar story in the bible!

There's a badass scene in the Bible where David kills a giant with a sling.

Not to be confused with the book, My Immortal