Note the stones between ranks and in hands....

i don't know how much we should base our ideas on ancient artistic depictions but look at the slings :
Greeks depict the two cords while the Assyrians depict one cord
could it be differences in sling types/ grips (wide vs narrow) / artistic preference?

Bilder wie auf der Trajan Säule sind schlichtweg nicht möglich . Und auch nicht sinnvoll. Wir haben es mehrfach versucht. Sie tragen ihre Steine nicht in einem Mantel . Und in der selben Hand noch einen Schild. Das funktioniert nicht.

Jaegoor, that is a great illustration!  Looks like a portrayal of the Philistine champion Goliath from the Bible.  Any ideas of where the painting is located? I can't read the Latin, but a higher resolution image might be helpful.

Of course one of the most prolific sling-using-armies was that of the Inca empire, and pre-Incaic civilizations too.  Protective padding of cotton and wool fabric as armor, clubs and spears for close range use, and the sling as the principal ranged weapon.  There are stories from the siege of Cuzco where red-hot sling stones were wrapped in pitch-soaked cloth.  This bundle would immediately begin to smoke and smolder, and then it would be slung, which would cause it to burst into flame due to the oxygen rushing in from the velocity of the hurl.  Such missiles were used to burn the thatched roofs and encampments of enemies, in this case, Spaniards and their allies in the capital.

In modern-day riots and insurrections, the vast majority of missiles are hand thrown... Nonetheless, with tear gas cannisters, baton rounds, and various counter-protesters thrown in, particularly in places where the sling remains in use, the sling turns up every now and then, although the sling shot seems much more common.

These dated images of the mob violence in Tahrir Square, Cairo show the extemporized protective headgear and shields--even pavises-- primarily to protect against hand-thrown stones.
http://www.telegraph.co.uk/news/picturegalleries/worldnews/8301665/Protesters-im...