I have been reading many of the posts here about the casualty rates from sling men.  
 
I would like to make some observations from reading them.
 
Armor in ancient and medieval times was very expensive, often soldiers did not have them. Furthermore, many of these armies had poor shield discipline. As such in these battles, a slinger would be much more effective then many give credit for here.  
 
So what happens when a mass of poorly armored soldiers with bad shield discipline are met by 100 slingers firing every minute a 1000 shots straight into them.  
 
 
A slinger is some ways is relatively cheap compared to other soldiers, as he does not need horses, armor or metal, which was expensive. He can often be recruited locally e.g. shepherds.
 
Even accepting that most here seem to assume that about 1% of the shots are effective. Assuming the slinger came with 100 shots into battle, he takes out one person. In comparison a pikeman overall has a 50/50 chance and takes out half a person.
 
It appears to me that a slinger has a high benefit/cost
 
 
 
 
 

Quote from David Morningstar on Jul 4^th, 2012, 2:59am:

 
Another point that follows on is that even a properly equipped army with disciplined troops because it needs to move in formation would be slowed down and restricted by slingers (and/or archers) even if they did not take out too many of the opposition forces.
 
 

Mauro Fiorentini
 
Quote:

 
Armor is expensive; many armies could not afford it for all their troops. Often only notable people had them, for example, many of the troops in Anglo-Saxon England  had no armor for this reason another would be the Scottish army under William Wallace, which had very poor-quality  armor.  
 
Interestingly sometimes it becomes almost a  badge of honor not to have armor for example, as Julius Caesar's records in his account of the Gallic War when the Gauls went into battle naked.  
 
Another point is that armor slows you down. The Mongols as time got on could certainly afford it but limited their armor because they did not want to sacrifice speed and mobility.
 
Shield discipline would very much depend on training.  I suspect that many of the medieval levies were not well trained.  
 
 
 
 

It's true that discipline and armor varies widely over time. I remember reading that in the middle ages, when the mounted knight reigned supreme, that infantry were eschewed almost universally. I was reading an article about the longbow and Britain and the authors mentioned that in a couple of instances the French cavalry trampled their own men in their haste to get to the battle. That's how little esteem and training went into the infantry. The British, the authors argued, were the first to really "rediscover" infantry's value and that was largely because they had a smaller population. I think there's evidence of using the sling at that point, but mainly in the context of sieges.  
 
I also like the point about lightly armored armies. But we tend to have better sources from the professional disciplined troops, which tend to have both better armor and better shield discipline.

Conscripted peasants, like at Hastings, usually didn't have armor or weapons either, they used farming impliments such as hoes, scythes, bush axes and pruning hooks.  The Celts also didn't use much armor, there are several accounts of Roman slingers picking their formations to pieces before the heavy infantry moved in to finish them off.  Most real troops were either issued arms and armor of various types and qualities or required to have and maintain certain items of arms and armor.  
 
There were "ad hoc" armies where large groups of a local population would form war bands or battle groups or militias to defend or attack, the amount and quality of armor and training would vary wildly from man to man and example to example.  Greek Spartans are an example of the better armed and trained, the peasants at Hastings, whose main job seemed to be to stop arrows aimed at the housecarls are am example of the poorer.

This question was discussed by R. Gabriel in one of his books (I referred to them in this thread:
 
http://slinging.org/forum/YaBB.pl?num=1257730993/0
 
Gabriel's point was in fact that the lethality of slings and arrows is not that high. He gets that slingstones and arrows are high lethality weapons, but notes that in no ancient battle was the main line of heavies mowed down. So Gabriel's assumption is that missile effectiveness is reduced by armour and shields-- probably considerably so. As confirmation of this, we might turn to Xenophon, who in his Anabasis provides an eyewitness account of what it's like to fight against slingers:
 
[3.3.6] After this they took breakfast, crossed the Zapatas1 river, and set out on the march in the formation decided upon,2 with the baggage animals and the camp followers in the middle of the square. They had not proceeded far when Mithradates appeared again, accompanied by about two hundred horsemen and by bowmen and slingers--exceedingly active and nimble troops--to the number of four hundred. [3.3.7] He approached the Greeks as if he were a friend, but when his party had got close at hand, on a sudden some of them, horse and foot alike, began shooting with their bows and others with slings, and they inflicted wounds.
 
-- wounds, and not fatal casualties. I also would mention Thucydides 2.81.8, describing a campaign in W. Greece in 429:
 
 
 But when the barbarians in their flight broke in upon them, they took them in and uniting their two divisions kept quiet there during the day, the Stratians not coming to close quarters with them, because the rest of the Acarnanians had not yet come to their support, but using their slings against them from a distance and distressing them; for it as not possible for them to stir without armour; and indeed the Acarnanians are famous for their excellence in the use of the sling.
 
 
The assumption seems to be that if you wear armour, you're actually somewhat safe from sling sniping. My own view is that the battle winning, ground-occupying, assault infantry which closes with the enemy and kills him or drives him off are, by nature, pretty heavily armoured (this also comes from reading stuff in another forum, fioredeiliberi.org); light infantry often runs off. (Caesar doesn't, I think, describe naked chaps in battle; that's Polybios in book 2, about Telamon. Caesar's opponents are pretty heavily armoured, if the archaeology is indicative).  
 
So: body armour (of any type: plate, or flexible— mail, linen, padded), helmet, and shield, are sufficient to greatly neutralize missiles of any type (slingstones, hand thrown stones, arrows); as Morningstar often observes on this forum, the real killer is the javelin, which has enough mass and punch to give even the armoured chaps problems. I'd add that the other way of neutralizing missile troops is another line of missile troops, which produce the characteristic pre-battle skirmishing between lights-- described by Thucydides in Sicily, or by Polybios at Cannae-- inconclusive.

Sorry for the third post: in the thread i quote above, I mentioned the battle of Sphakteria (425 BC), where 10,000 men (including a good deal of highly specialized missile troops) pelted 400 Spartans over several hours, and killed 100 + of them (and admittedly wounded most of them-- but by the end of the day they were still combat capable).  
 
You can calculate the various ratios; but if the figures are accurate, it might be a case of 50,000 projectiles killing about 120 men, so perhaps 400 projectiles for every KIA. The calculations may be off, but give an idea-- and this is in pretty optimal conditions (flanking fire, no competing line of skirmishers), and over hours. And, by the way, the Spartans were a garrison, and not wearing heavy kit-- no body armour, floppy "bush hats" rather than helmets, just their shield.  

Although many soldiers in ancient and early medieval armies would have lacked armour, by the late middle ages few soldiers in Europe would have lacked armour completely (or proper weapons), which probably had something to do with the slings disappearance from the European battlefield.
At the battle of Najera in 1367, a huge Franco-Spanish army that included 4,000 Spanish slingers was routed by a much smaller English force, and suffered dismal losses.
http://en.wikipedia.org/wiki/Battle_of_N%C3%A1jera
As far as I know, Najera was one of the last times that mass numbers of slingers were deployed in a European battle.

As for armor, look at the clothing of the time, the leather was not split or shaved as it is today, so a leather vest or coat would would be much thicker and provide some pretty good protection, as would a leather hat.  The linen, felt, and wool clothing was much thicker than what is worn today, it also provided good protection.

Vegetius mentions that flexible armour is no protection against sling stones.

I'm quite sure that 12mm leather would stop,ricochet,contain a 30-40g lead gland.It would probably still make a bruise under,unless a good subarmalis is worn underneath it.If a 105g lead bullet with DEXAI written on it is in the equation...then the wearer is having a bad day.

maces,clubs and slings deal blunt damage,rather than piercing. A huge hit from a blunt weapon can transfer shock,concussion through armor,which can cause wounds or be even lethal,if the spanish guy gets a big hit on the helmet.Armor will just bend ,but still can cause a bad bruise,broken rib,cracked skull,K.O. etc.
No penetration involved,with stones and ceramics,and lead bullets were usually too small weight and lead too soft to penetrate.Andeans probably didn't used lead projectiles,at least i know.
The nobels might had used gold bullets,as i read from Masika's post if i remember correctly,but not sure bout that one though. Even tempered armour piercing arrowheads have a hard time piercing plate armor,shot from a warbow.