Lloyd explains what made the sling — which bears little resemblance to the modern sling-shot — in many ways better than the bow:
The sling is very easy and cheap to make. Most in the past were made of leather, some being rush or twisted cord. The amount of material needed is minimal, and anyone who knows what a sling should look like could make one in a few minutes. Bows take far more materials, and rarer materials too. Bows take more maintenance, can break when you fall over, take far more time and skill to make, and are more cumbersome. A slinger could carry half a dozen spare slings easily, while an archer would worry about damage to his one bow.
A sling might be carried without ammunition, with the thought that some could be found when needed. Bows take very specialist ammunition which needs to be well-made in advance, and maintained. An archer would want to recover as many of his arrows as possible after use. Arrows are expensive, and can warp in damp weather. Arrows are long things need to be carried in an awkward quiver which flops about as the carrier runs. A pouch of sling stones can be a neat bundle, a more manageable load.
It is well known how bows are affected by weather. Battles have hinged on whether one side, with superior archers, has been able to make use of its bows effectively. Even quite light wind will blow arrows off course badly, and rain will spoil bow strings, and drag arrows down from the air. Slings, while still adversely affected by wind and rain, suffer not nearly so much from bad weather. This may explain why armies with archers often valued having slingers as well.
Slingers are generally more mobile than archers. They find it easier to shoot on the move and have the great advantage of needing only one hand to shoot, which allows them to use a shield in their free hand to protect themselves. It is possible to load a sling one handed, and I find that the best way to do this is to kneel down quickly and use the ground as a third hand: put the sling down letting go of one string, get a stone, put the stone in the sling, then pick up the sling again by the loose string and stand up again. While doing this, you would want to have a shield for protection, since you have to take your eye off the enemy. One can sling while kneeling, but the shot will not be as powerful or accurate. Archers in ancient armies often wore armour; they needed it more. While some archers did sometimes carry shields, these could not be used for parrying while shooting. All this may explain while slingers were often deployed as skirmishers on the field rather than in huge formations.
Arrows can be seen raining down upon an enemy, and even when they are flying on a fairly flat trajectory, are visible to an enemy expecting them. Sling stones are much more difficult to see in flight, especially from a distance. It is also more difficult to judge which way they are going, as they are seen as a dot rather than a line. Sling bullets, which are cast lead shot, are especially difficult to see. It has been speculated that this difficulty of seeing the stones in flight might be both advantageous and disadvantageous. A cavalry formation charging into a shower of arrows, might be broken up or slowed down when the riders look up to see the arrows and try and avoid them. Slings would not break up formations this way so readily, but might gain from allowing less evasion.
One advantage that the bow has over the sling is that bows can be used more easily in deep formations of troops. Archers could angle their bows to shoot safely over the heads of their fellows in front of them. While slinging over the heads of friendly troops is possible, it is much more dangerous and was seldom attempted. In later periods, when fortifications had slits for shooting from, bows and crossbows were better suited to this than slings.
One further comparison with the bow which should definitely be made is that of the skill needed to operate the weapon well. A man might be taught how to use a bow to a useful standard quite quickly. Judging the range of an oncoming line of troops might be difficult, but at least the archer could shoot an arrow well enough to make it look threatening. Slings are different. To get good range with a sling takes practice. With one of my slings, I might sling a stone a bit bigger than a golf ball only seventy yards or so. Ancient slingers with much more skill than me could get a stone over twice this distance. There are peasant boys in Africa who use slings to herd sheep and goats. They sit in the shade of a tree, and if they see an animal straying, they sling a stone in front of it to scare it back into the flock. To gain this sort of skill, I am told it is necessary to start young. Good slingers in antiquity were in demand. Particularly famed for their skill with slings were the men of the Balearic Isles (islands in the Mediterranean including Majorca, Ibiza and Minorca). These slingers practised their skill from a very early age, their original purpose being to hunt and to scare pests. Their skill brought them employment from the Romans.
Both Roman and Greek writers say that the sling could out-range the bow. The advantage of range is repeatedly stressed. This could, it seems to me, be because the sling had a greater effective range, arrows losing their power to air-resistance after a while, and falling out of control onto their target, whereas a sling stone might build up a more dangerous speed just from falling. The effective range of slings seems to be in excess of 360 yards. Assyrian reliefs show slingers attacking cities from further away than the archers. Perhaps this is because the archers were used to shoot straight at defenders on the walls, while slingers dropped stones into the city, or perhaps it is just another clue to the greater range of slings.
Writers tell of the terrible wounds that slings would inflict, especially bullets. The Romans developed a special pair of tongs designed for getting bullets out of people. Arrows, unless barbed and deep in the victim, are easier to extract. There was also a belief, presumably false, that sling bullets got white hot as they flew through the air. Julius Caesar writes about clay shot being heated before slinging, so that it might set light to thatch.
The power of slings is famous. When iron plate-armoured Spaniards went into South America against the Aztecs, only the slings of the Aztecs were feared. The stone-tipped arrows would glance off or shatter against the armour, but the sling stones would damage the Spaniards by sheer smashing force. I have demonstrated the power of a sling by slinging a lump of chalk rock against a large tree. The stone does not bounce [off] the trunk. Instead, where the stone impacts, a cloud of dust appears, and wafts away, being all that remains of the rock.
Slings are, of course, still popular in the Middle East.