The sling seems to be something of an under-rated weapon. Many wargamers see the bow as the ancient missile weapon, used by competent soldiers, whereas ignorant barbarian peasants use slings. In truth, the sling is an effective weapon, and has many distinct advantages over the bow.
The item itself is much smaller than the bow, easier to carry and conceal. I have made a few slings, and find that when folded and bound up by their own strings, they become a soft tiny bundle which can be slipped into a pocket and forgotten about. To do this with a bow, one would need special pockets.
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.
Sling bullets, or glandes, were made of lead, and were usually cast in moulds. The flash from casting can sometimes be seen on excavated examples. Thousands of sling bullets have been found. A group of sling bullets from Corbridge on Hadrian's Wall shows signs of hammering. Interpretation of finds in archaeology is, of course, a problem. None of the Corbridge finds showed any signs of impact, so it is possible that these are not sling bullets. Most bullets approximate to a shape which is variously described as plum, almond, or bi-conical, depending on which archaeological report one is reading. The size of bullets varies remarkably, with 30-35mm being fairly typical. One collection of bullets from Windridge Farm, St. Albans has examples weighing from 28 to 78 grams. The heaviest from Vindolanda is 118g, and of the Corbridge examples, 142g has been recorded. The heaviest bullet from the Near East is 185g. A low to mid-range bullet from the Windridge Farm collection is probably the best indicator of a typical sling bullet. I wonder if the larger bullets might not be for staff slings or artillery.
There are two principal types of sling bullet, which, after much imaginative thought, archaeologists have called Type 1 and Type 2. Type 1 is more common, pointed at both ends, with an oval cross-section. Type 2 is acorn-shaped, literally - it looks like a model acorn in lead. Some spheres of lead have been found and tentatively interpreted as sling bullets, but this is yet to be established.
Facimile Roman sling bullets, or glandes. These are castings in lead made from actual glandes found on a military firing range in Northumberland. They are all type 1 stones. The larger weigh 74g (2.61oz) and the smaller weigh 39g (1.38oz). They are hard and dense, and one falling on your head from even a short distance would hurt a lot.
Names, symbols, and messages were often cast onto sling bullets. Sometimes the name of the maker was on the bullet, sometimes the owner's, or the owner's unit, sometimes his enemy. The messages are interesting. The Greeks especially went in for these. They said things like "Take that!" and "Megacles hit you". The messages could be quite up to the minute, since the bullets were often cast on campaign.
In Britain, sling bullets were used mainly in the iron age and early Roman period. The Greeks were the main users of lead shot. I have come across one reference to bullets being used at the battle of Marathon (Greeks 3, Persians 0, home win). Some examples were excavated at the site of the battle. Xenophon mentions Rhodian slingers being asked to volunteer to fight in the Greek army against Mithridates. These slingers, who did not fight in formal units, were said to use bullets which gave them twice the range of the less-skilled Persian slingers who used fist-sized stones.
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.
Sling units were employed in the auxiliaries in the Roman army in the Republican period. The use of the sling was part of the basic training of all soldiers, who were also trained to throw stones up to one pound in weight by hand, a method which was considered more readily employed, which I can well understand - it takes a fair few seconds and preferably both hands to get a sling out and ready. Pompey in the civil wars favoured the use of very large units of slingers. They were used beside archers, at sea, and in sieges. Scipio used them against elephants, and Caesar comments that the sling was particularly effective against them.
Contrary to popular belief, the sling is not whirled above the head several times, building up speed, before the stone is released. A sling might be whirled a couple of times slowly if the slinger had time, to get the feel of the weight of the stone, and while sizing up the target, but it is one big movement which sends the stone on its way. Anyone who makes a sling will find that they can whirl the loaded sling round and round far faster than they can cope with when it comes to releasing the stone. Also, slings are generally used over-arm, like bowling a cricket ball, rather than side-arm, like skipping a stone across water. A side-arm action allows for greater accuracy regarding elevation (up and down) but less regarding windage (left and right). A slinger who makes an error using a sling sideways is in danger of hitting his friends to his left or right. A slinger slinging over-arm will err only to sling into the ground in front of him, or over the heads of his foes; and he needs less room to sling, and can sling from behind a wall.
A good sling stone makes a big difference to the range and accuracy of a sling shot. In my experiments, I have noticed angular stones turning at surprising angles in the mid-flight, and I have always been more accurate with rounded stones. Stones found on beaches of certain types of rock have proved, though rounded, too rough and grippy; they leave the sling too late and crash into the ground a few yards ahead of me. Stones grubbed up from river banks have often been muddy and have sometimes slipped out of the sling early, flying high in the air, again to land a few yards ahead of me. I have never accidentally slung backwards - there is always some forward motion to the stone. Dry smooth and rounded stones are best, which goes a long way towards explaining why hoards of good sling stones have been found in iron age hill forts like Maiden Castle. It also explains why people went to the trouble of making fired-clay sling ammunition, which is also found in Britain. I have seen Assyrian sling stones of about 2¼" diameter, which were carved into a rough sphere.
Some stones I selected for use with my hand sling, of about the right size. Although they are all about the same size, they vary in weight from 105g to 160g (average 131g), so the heaviest is over half again as heavy as the lightest, suggesting that stones, even those as these found in the same place, are inconsistently dense. They are not too rough and grippy, but not too slick either.
There does not seem to be one way of holding a sling. The grip I favour has one string wrapped around my ring finger, and the other held between my thumb and centre joint of my forefinger. Ancient depictions show other holds. Assyrian reliefs show one string apparently looped around the whole hand, and the other held by all four fingers, with the end poking out of the closed hand. Rhodian slingers have been pictured with the fixed string tied round the wrist. With all these grips, one string is held and released at the top of the arc, releasing the stone, while the other string remains fixed one way or another, to the slinger.
My preferred sling grip. The fixed end is attached to my ring finger, seen here done with a loop, and the thumb grips a knot on the free end against the side of the index ginger.
The sling strings held tight, ready for slinging. The strings are about parallel.
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 of the trunk. Instead, where the stone impacts, a cloud of dust appears, and wafts away, being all that remains of the rock.
No actual ancient slings survive. Sling stones are found in a variety of contexts, indicating that they were used for hunting very often, and not just for warfare. Civilians in Roman times were allowed to use slings when other weapons were forbidden. Lead bullets turn up only in military contexts, suggesting that they were used for warfare only.
The use of the sling declined during the Roman Imperial period and this decline continued after the fall of the Empire. The last outpost of sling use was Britain. The reason for this change is probably the most usual reason for all military changes in the pre-gunpowder period: fashion. At first, there were proud units of specialist slingers who gained great skill with the weapon. Later, the sling became used more and more by "amateurs" as a secondary weapon, and it began to be regarded as a barbarian weapon beneath the dignity of professional soldiers.
To conclude, the sling is a potent weapon which deserves greater respect from wargamers. It might be used in games other than ancient battles. Perhaps in a post-holocaust setting, soldiers might meet some apparently unarmed people, who later attack with slings in the woods, and then dodge away into the trees, while the soldiers waste their precious ammunition in gun-fire retaliation. Slings are spiffy.