This is a good read!
The Roman army under Marc Antony fights the horse archers of Persia. I think I've read another version of this that gives more details. I'll see if I can find it.

In this way he met Pacorus in Syria Cyrrhestica and conquered him. For when he had not prevented them from crossing the river and had not attacked them at once after they had got across, they imputed sloth and weakness to the Romans and therefore marched against their camp, although it was on high ground, expecting to take it without resistance. But when a sally was suddenly made, the assailants, being cavalry, were driven back down the slope without difficulty; and although at the foot they defended themselves valiantly, the majority of them being in armour, yet they were confused by the unexpectedness of the onslaught and year stumbling over one another and were defeated by the heavy-armed men and especially by the slingers; for these struck them from a distance with their powerful missiles and so were exceedingly difficult for them to withstand. The fall of Pacorus in this struggle was a very great loss to them; for as soon as they perceived that their leader had perished, although a few men zealously fought for his body, yet when these also were slain, all the rest gave way. Some of them desired to escape homeward across the bridge and were unable to do so, being cut off and killed before they could reach it, and others fled for refuge to Antiochus in Commagene.
Cassius Dio XLIX.20

Just shows how important the slingers are. this was an argument between me and my dads friend, he thoght that a sling was usless.... ;D HA showed him!

all i have to say is slingers so rule!

I wonder what sort of ammunition they were using? Glans-type lead slugs, or big rocks?

Oz

I think they were casting lead glandes. I will look at my sources today to make sure.

Plutarch, life of Antony, specifically mentions lead bullets during Antony's campaign (against the Parthians, not Persians: Iranians who took over the Eastern part of the Seleukid empire in the late C2nd BC).