Just some thoughts inspired by W. K. Pritchett's wonderful chapter on slingers, in Greek State at War, chap. 5 (on Google Books, BTW).

Slingers belong to the category of light-armed (psiloi in Greek, expediti, etc, in Latin). How do you employ them ?  No doubt the sling served in "small" or "skulking" war for the communities that specialized in its use, for instance the Cretan cities, the Akarnanians, Achaians; Balearic islanders. But in the sort of operations that turn up in our sources (literary, concerned with big war), en masse, in corps of hundreds, sometimes thousands of slingers, archers, javelin-men, on foot. In sieges, they serve to manage space-- clearing space for assaults, making siege works difficult, preventing sorties (the siege of Same in 189 gives an example of this).

In the field ? The enemy is the heavy infantry, the mainstay whose presence holds ground, whose clash is honourable and decisive. WHen drawn up in tactically significant masses (say a box of men, 2000 men, 8 deep, 250 across,)-- a lovely target. Light missile troops contain them, force them to hunker behind shields and make them tactically immobile, make tactical movement difficult and costly (especially in terrain such as passes or river crossings). Even the Greek shield with specially added on curtain to protect the legs against missiles (visible e.g. on the reliefs from the Nereid tomb from Xanthos), or the big Roman scutum, will not make infantry movement under sling and arrow fire any easier.

The light armed corps' mobility means that the main counter, and indeed, main infantry job of closing for hand to hand fighting with edged weapons, is impossible (and failed attempts to close will prove very costly)-- this is true even for javelineers, who have to run up to reach javelin range. If heavy infantry is pinned (by terrain, by other infantry), it can be wiped out or forced to surrender. The earliest detailled descriptions of this sort of clashes are in Thucydides-- where both professionals of the javelin, and light-armed amateurs, inflict checks on heavy infantry; more examples in Xenophon.

What's the counter ? Another screen of light infantry; as Thucydides writes (for a battle during the Sicilian expedition, in 413 I think)-- light infantry drives off light infantry, each side wins and loses, before the serious stuff-- heavy infantry work-- starts.

How do you deploy e.g. 500 slingers ? They can't be shoulder to shoulder, obviously, whatever the slinging style-- perhaps 2.5m to 3 m between each slinger, so the "firing line" is a widely spaced line of men. Is it only one line ? Or is there a second line of firers, disposed in a checker board ? Or even several lines ? A loose mass of men, the rear elements firing above the heads of the others, towards the enemy heavy infantry.

The fun begins when one line of slingers meets another line, within effective range (say 150-200m ?). The contest is no longer about area fire into a mass target, but plinking at individual slingers on the other side, in sharp shooting duels. Slingers drop to reload and offer smaller targets, jump about to offer difficult targets, and try to conserve ammunition, to outlast the other side. What then happens ? Sometimes, one side is very much beefed up, or ione side panics and is drawn to loose off all of its ammo: this happened to the contingent of archers in Athenian service, during their invasion of Aitolia in 427 BC-- in a battle against Aitolian psiloi, and without good light infantry support (since the Athenians had marched on without wiating for Lokrian allied support, which would have included psiloi), the Athenian archers were outnumbered by the Aitolian stone throwers, their commander was killed by a stone, the archers ran out of ammo, the Atiolian stone throwers drove them off, then did for the Athenian marines. More usually, both sides of light infantry draw out each others' sting, then move aside.

Still nerve wracking for those who had to fight in the slinging line, no doubt.

I would deploy slingers as far forward as possible to maximise the advantage of range. First targets would be would be large blocks of men because they are easiest to hit. Concentrate on the less experienced or perhaps less well equipped infantry and see if you can rout them or at least knock them into serious disorder.

When the range is close enough to throw more accurately, use concentrated volleys to attack the enemy slingers and archers. They will be well dispersed and moving as much as possible so it would be necessary to pick a smallish area and saturate it with shot. Move the aim point around for each volley.

As the range closes you have pull the slingers back to the flanks and deploy your javelin throwers instead (or swap from one weapon to the other if your light troops are multi-role)

After the infantry have made contact the slingers of both sides would be quite close to each other at the flanks. You would have to hold your ground in a slinger-vs-slinger shootout. The side that lost that duel would then find their infantry flanks exposed to raking close ranged shots while they are trying to fight the enemy infantry.

It apparently stands to reason that slingers would not be able to stand shoulder to shoulder in close formation. Yet that is precisely what the Assyrian relief sculptures featured on our home page show. Is this just artistc licence or did they really stand so close and throw overhand style and in unison?  Please comment as this has really puzzled me for some time.

Aussie-- I suppose that in sieges, the point is massed fire, to clear defenders off the wall ('managing space' through the application of harm), and slingers can fire massed from behind field fortifications (could they even manage "indirect fire", with ranges called and fall of shot observed ?). But there must be practical limitations, even for overarm fire with no windups-- if only because of the risk of fouling your neighbour's shot ? Time for some experimentation !

I reckon the real Greek style of slinging was either a figure-8 or a simple overhead throw like this: http://www.youtube.com/watch?v=gTT78jJuFuU

With the rotation and throw in the vertical plane you are much less at risk of hitting or tangling up with your neighbour. It is also accurate left-to-right, leaving only the range as a variable.

There is a vase painting that shows a man in "starting" position, not in the well known "pouch before the eyes" aiming position, but simply with outstreched arm to aim, and sling holding arm held out back, with sling hanging--  perhaps the representation of a simple overhead, "whipped" throw, rather like a javelin throw.

That's the one ! The man wears a soft woolley hat, and a skin serving as a rough shield.

If that drawing is accurate why is the lefty slinger’s right leg appear facing the other way, or is he actually a righty ?