Disclaimer: I don't personally know how to use a sling, because they are hard, and that is the point...
Of all the weapons that appear in the D&D game, the humble sling may be the most mis-represented. This all probably started with G.G. making slings the only ranged weapon available to Thieves in AD&D, with an eye towards his own vision of Thieves as an 'urban' class -- despite the fact that slings are a very pre-urban weapon and traditionally associated with rural shepherds.
This carried over through many versions of D&D, and, mostly from the view of keeping the original Thiefly weapon selections the sling was classified as a "Simple" weapon in modern (3.x and later) D&D and given very low accurate range and low damage output. All of these things are just...wrong. The weapon described in the D&D game is clearly not a sling, it is a "slingshot" -- which is extremely anachronistic to most D&D settings since the slingshot did not (and could not) exist prior to the invention of vulcanized rubber (1839).
First, let's address the modern classification as "Simple" weapon. Excluding weapons which are simply thrown, ranged weapons in D&D basically fall into three categories: crossbows, bows, and the sling. The crossbow is fired by pointing it at a target, siting along the stock, and pulling a trigger -- the simplest possible method of attack. This is a simple weapon. The bow is similarly intuitive, knock, draw, release -- though it requires more practice to do accurately. This is a martial weapon.
Accurate use of a sling requires the user to stand 60-degrees off from their target (rather than pointing strait at them), nest the bullet, rotate multiple times to generate speed, then release at exactly the right moment in the arc to send the bullet towards the target. Because of the rotation of the sling, any minor variation (early or late) in the release timing will result in missing the target entirely. This is not intuitive, and requires much more training and practice than accurately using a bow. While the sling may have been "common" in ancient warfare, it is by no means "simple". In sufficiently early settings where they were in major use the sling might be considered a "Martial" weapon, but, by virtue of difficulty of use, should probably be classified as "Exotic".
Secondly, there is the issue of range. Pathfinder lists the sling's range as a 50-ft. range increment (compared to 100 ft. for a longbow or even 60-ft. for a shortbow). Procopius's "Wars of Justinian", the writer implies that the Roman slings had greater range than the bows used by the Huns, and other ancient writers repeatedly stress the slings advantage of range over bows. Modern tests have shown that, with a high trajectory, a sling can have an effective range of more than 1200 feet (a range only matched in testing by high draw-weight, high trajectory composite bows). Weapons in Pathfinder can be fired a maximum 10 range increments, which, if we go by the 1200-ft. number, puts the range increment for a Sling at 120-ft. (rather than a mere 50-ft.) which fits with accounts of it out-ranging a bow.
Then there is the matter of damage-output. 3e D&D and Pathfinder list the sling's damage as a meager 1d4 (less than a thrown club -- which is less dense, fast, or aerodynamic than a lead sling bullet). Everyone is familiar with the story of David and Goliath (a boy taking out an armored giant with a single well-placed sling bullet) -- but that can be easily written off as a critical hit / divine smite / etc. However, the late Roman writer Vegetius gives an argument that slings, in general are more devastating than bows (in addition to the better range), writing in his "De Re Militari":
"Soldiers, notwithstanding their defensive armour, are often more annoyed by the round stones from the sling than by all the arrows of the enemy. Stones kill without mangling the body, and the contusion is mortal without loss of blood."Numerous accounts of both men and beasts (lions, bears, etc.) being slain by a single (long range) shot from a sling exist. Implying that the weapon is at least as deadly as a bow or crossbow. Both bows and crossbows deal 1d8 damage in D&D, so it stands to reason that a sling bullet should deal at least that much.
Further, Vegetius writes:
"There is the greater reason for instructing all troops, without exception, in this exercise, as the sling cannot be reckoned any encumbrance, and often is of the greatest service, especially when they are obliged to engage in stony places, to defend a mountain or an eminence, or to repulse an enemy at the attack of a castle or city."So, you've got a weapon that weighs practically nothing, fits in your pocket, has near infinite readily available ammunition just lying on the ground (though cast lead bullets are better), can be rapidly manufactured from simple materials, has range better than a bow, and is just as deadly (if not moreso) than other available ranged weapons. Why would anyone use anything else?
Because (and this gets back to the first classification point), it is hard to learn to use. Slings are an Exotic weapon, that takes extra training (read Feat or Proficiency slots) to learn to use effectively. Any random yokel (Thief, Wizard, etc.) can't just pick this thing up and use it with any level of success.
So, for you Pathfinder playing people, here is the corrected entry for the weapons table:
|Cost||Dmg (S)||Dmg (M)||Critical||Range||Weight||Type||Special||Source|
If you need mechanical justification for why it's "Exotic" despite having damage output set at the same as a bow. (1) The range is longer. (2) It costs nothing and weighs nothing (which is a huge advantage in resource-strapped campaigns).