At the beginning of Malcolm Gladwell's David and Goliath, the author addresses the deadliness of slings as terror weapons in an adroit, redesign-encouraging passage:
"Slingers had a leather pouch attached on two sides by a long strand of rope. They would put a rock or a lead ball into the pouch, swing it around in increasingly wider and faster circles, and then release one end of the rope, hurling the rock forward. Slinging took an extraordinary amount of skill and practice ... but in experienced hands, the sling was a devastating weapon. Paintings from medieval times show slingers hitting birds in mid-flight. Irish slingers were said to be able to hit a coin from as far away as they could see it. And in the old testament book of Judges, slingers are described as being accurate within 'a hair's breadth.' An experienced slinger could kill or seriously injure their target at a distance of up to 200 yards ... imagine standing in front of a major league baseball pitcher as he aims a baseball at your head. That's what facing a slinger was like. Only what was being thrown was not a ball of cork and leather, but a solid rock."
There's more; it's a very good book. Gladwell's point is that in the Goliath vs. David conflict, David is actually the far deadlier combatant. This detail is also added: "... a typical size stone, hurled by an expert slinger, at a distance of 35 meters, would have hit Goliath's head with a velocity of 34 meters per second, more than enough to penetrate a skull ... in terms of stopping power, that is equivalent to a fair-sized modern handgun."
This is all fascinating, and in the light of the above many DM's would be encouraged to alter the amount of damage from a sling from a mere d4 to a d6 or a d8. Some might even be encouraged to create a possible rule where an accurate hit from a sling would have a 1 or 2 percent chance of death for the opponent ... because that is the way that things like the above are usually interpreted.
But note that, while Gladwell makes his point, he is careful several times to qualify the effects as coming from an experienced slinger; that use of the weapon was extraordinary; that the hand-gun stopping power was something an expert slinger could accomplish.
The paintings of slingers hitting birds were not intended to be representational of every slinger, everywhere ... they were examples whereby the painter was IMPRESSED by the event. The description of Irish slingers hitting a coin is the sort of thing that either the Irish would say in order to boast of their accomplishments, or something the opposition would say as a rumour sweeping through a camp. So it would be true with the Book of Judges, which anyone would have to admit was a set of propagandistic tales of the first order.
I don't doubt that some slingers could do all of the above. There's no question at all that a slinger could kill at 200 yards. It is not the deadliness of the sling that is in question, it is whether or not the average, non-expert, non-extraordinary soldier could pick out a specific target at 200 yards and, with a sling, hit that target so squarely that it would react as though it were struck by a bullet.
And the answer to that is no, certainly not.
There is a tendency to presume that when anything is described in an historical context about an event or a practice, that this is a representational description. It would be like Alpha Centaurans reporting on events they had seen on Earth and writing down the words, "Humans are able to throw balls of cork and leather at 100 miles per hour!" (or whatever unit of measurement they would be using). Not ALL humans, obviously! Just some humans.
So when you read about a sling killing at 200 yards, does that mean a sling aimed at something 200 yards away and then consistently and predictably hitting target after target at that distance? No, it doesn't. It means we found one body lying in the dirt, with a stone in its head, and someone said, "Wow, those slingers are two hundred yards away ... that's amazing!" Chances are, in the heat of battle, the slinger probably wouldn't have known they had killed that target; they're just slinging like mad, keeping up a steady volley of stones in the direction of the army that's coming on, and hoping they do the most damage possible.
One reads the same sort of assumption error when long-bows are described as killing targets at 800 yards ... and then a DM vastly increases the range on their weapons in the game - because that sounds logical. It's bee proven that a bow can be accurate at half a mile.
That's "can be" ... but does that mean "will be in this specific instance"?
Because, really, it's the will be that matters.
Here's something else to consider, while we're pondering the above event between the slinger David and the opponent Goliath. Off hand, with all the memory you've got, how many recorded encounters can you recall between a single slinger and a single target where BANG! ... one hit, and the target died?
Myself, I can only recall the one.
I have a rule in my AD&D version of the game that if you roll a natural 20, the damage is doubled. Thus, the sling would do 2-8 (d4x2) rather than 1-4. The player then rolls another d20, before doing damage, and if another natural 20 is rolled, then the damage is tripled (d4x3). Another d20 is rolled, and another natural 20 causes the damage to be quadrupled. And so on. As long as sequential 20s are rolled, the damage is increased in kind.
The chances of rolling two natural 20s is 1 in 400. The chances of three, 1 in 8000. The chances of 4, 1 in 160 000.
I have had four 20s rolled during a combat at my table. So the odds I haven't seen are 5 20s, or 1 in 3.2 million. But once the four 20s were already rolled, the chances of that had been reduced to 1 in 20.
With all the slings that have been fired in all the years of history since the Upper Paleolithic, we have this one account. Gladwell describes it in his book almost as if it were a foregone conclusion ... but even the people who witnessed it chose to describe it as the will and hand of GOD, and not an obvious result; and that particular way of looking at that particular event has not been shaken in the 3,000 years of history since David, even though for all that time slingers have been hitting birds and things within a hair's breath and coins as far away as they could be seen. Where is the written account of someone in the Roman era saying, "Yes, well, don't stand facing some guy with a sling, because that's pretty much a guaranteed death." There isn't. Slingers - with an S - are terribly, awfully dangerous. A slinger - one - well that pretty much depends on the individual, and whether or not god is on that individidual's side.
I don't disparage Gladwell's book. It is a spectacular book; you're an idiot if you don't buy it and read it. I have every reason right now to believe that Malcolm Gladwell is the smartest man on the planet. Gladwell would be the first to disagree with me. All I want to say is that where we are adding up the logic of things, we have to be careful not to presume that because A is true, B is true as well; because we don't have any evidence that B is true. We haven't actually done any groundwork on the effectiveness of slings used on a battlefield where combatants are massed in the tens of thousands.
We have done a lot of study, however, on the use of rifles on such battlefields ... where we can demonstrate that YES, rifles in the hands of experts can be counted on to hit and hit and hit targets up to half a mile away. And guess what? In actual combat, it's extraordinarily doubtful that this accuracy accounts for much. There's too much mayhem and distraction and targets that can't be seen and a motivation not to actually shoot at people (people really would rather miss) to be certain that accuracy is about anything except a pissing contest performed by military folk on a firing range. I would like to see the accuracy obtained from two soldier in opposite, open positions, firing in each other's direction simultaneously at targets a foot and a half to the left of the opposite soldier. Now that's research.
David just happened to roll an awful lot of 20s all in succession, just enough to kill Goliath. I don't think the damage done by a sling needs to be changed at all.