A sling is a projectile weapon typically used to throw blunt missiles at the enemy. There are two kinds of sling, the shepherd's sling, and the stave sling. The shepherd's sling is almost certainly the one used by David to slay Goliath. It is the easiest to construct, carry and use. The stave sling is more important militarily, and can throw much farther and harder.
This is one of the most portable and least expensive personal weapons ever developed. Ancient armies probably used variations on it because so many men were already familiar with it. It is so lightweight that most soldiers can carry it as an auxiliary weapon.
One does not whirl it around one's head. One uses a shepherd's sling to make an overhand throw, using the sling to extend one's arm. This is relatively accurate, instinctive and quite powerful. One faces 60 degrees away from the target, with one's weak hand closest to the target. The coordinated motion is to move every part of the body, legs, waist, shoulders, arms, elbows and wrist in the direction of the pocket in order to add as much speed as possible to the stone. One releases the knot near the top of the swing, where the stone will proceed roughly parallel to the surface of the earth.
The clumsiest part of using a shepherd's sling is to regain control of the released knot-end. Conventionally, the loop is placed around the strong hand's thumb, or held by the two weakest fingers of the strong hand. Several stones are held in the weak hand. After the release, an expert will continue the motion. The pocket will catch around a stone held out with the weak hand, so that the knot end swings back to the strong hand retaining the loop. Just after the knot begins to swing, slightly before the knot reaches the strong hand, one drops or throws the stone toward the ground with the weak hand, starting into the next release. Some persons braid the knot around a weight to help perform this maneuver.
With this method, a skillful user can throw an aimed stone per second in a cyclic coordinated movement, until the weak hand is empty.
A classic shepherd's sling is braided from hemp or wool twine. Wool is softer, smoother and can be woven in colors, especially natural black and white. It is more comfortable for slings that are worn around the waist as belts. Hemp is stronger, and doesn't rot or stretch. It is both more accurate and more durable in wet climates. However, a loosely-braided hemp sling can make one's hands very sore until calluses develop, because hemp twine is harder, and more prone to pinch or splinter.
A shepherd's sling is quite easy to make by braiding, and braided slings are far more durable and accurate than slings constructed of leather, cloth or rope.
A typical pattern starts with four strands, roughly 4 meters long. The center of the strands is braided for about 12cm in a round soft sennit. This is doubled-over to form the thumb-loop. The round sennit is continued two-strands at a time from the root of the loop. When the sennit would reach the user's ankle when dangled from the hand, the pocket is made as a flat sennit. After finishing the pocket, one braids a round sennit two-strands at a time up to the other end, and finishes with a large round knot, such as a turk's head. The difficult part is to form the ends of the pocket, which should form the pocket into a cylindrical ellipse. However, perfection is not required to make a perfectly serviceable sling.
The classic military sling is a stave-sling. It consists of a stave, a length of wood, and a pocket, with a loop on one end that can slide from the end of the stave and release the glans. The other end of the pocket is nailed to the end of the stave. Normally the pocket is braided, usually from hemp or linen, sometimes of soft leather (see below).
Stave slings are extremely powerful because the stave can be made as long as two meters, creating a powerful lever.
Military slingmen used a lead projectile called a "glans" (acorn). A typical glans was an ovoid, or double cone made of lead, about 2cm in diameter and 5cm long. Usually, a glans would have propaganda molded or scratched into it: "Cretans are filthy vermin!" "All Egyptians are cowards!" "Romans Triumph Ever!" On ancient battlefields, glans are extremely common.
The stave sling is difficult to aim in altitude, so it was probably used for mass bombardment. The large number of glans on ancient battlefields support that theory.
The trebuchet is a scaled-up stave sling used to reduce fortresses.
Ancient art shows slingmen holding stave slings by one end, with the pocket behind them, and using both hands to throw the staves forward over their heads.