PHOTOGRAPH BY JOHN REID
it wasn't just Roman soldiers: In Xenaphon's March to the Sea, they were confronted with an enemy who was hitting them with similar weapons. So he asked around and found men who had learned to use the slingshot, and organized them to fight back.
more here oat Wikipedia
and here .at lloydianaspects has an excellent article on why the sling is such a great weapon and why it is underrated: it's not macho.
The sling seems to be something of an under-rated weapon. Many wargamers see the bow as the ancient missile weapon, used by competent soldiers, whereas ignorant barbarian peasants use slings.
In truth, the sling is an effective weapon, and has many distinct advantages over the bow. The item itself is much smaller than the bow, easier to carry and conceal. I have made a few slings, and find that when folded and bound up by their own strings, they become a soft tiny bundle which can be slipped into a pocket and forgotten about. To do this with a bow, one would need special pockets.
most of th stuff on line about slingshots are rubber type modern weapons, but this film shows how to make one fron scratch... literally, starting with getting fiber from leaves.
To use it, the loop is slipped over the finger and the knot is held down with the thumb. A stone is placed in the pouch and the sling swung over head. At the appropriate time the sling is swung forward in a throwing action and the knot held in the hand is let go. This causes the pouch to open and the stone to fly (hopefully) towards the target.