Ranged weapon attack rolls made with this ammunition ignore half and Cover. In addition, attacking at long range doesn't impose disadvantage on ranged weapon attack rolls made with this ammunition.
Sling bullets are used with a weapon that has the ammunition property to make a ranged attack. Each time you attack with the weapon, you expend one piece of ammunition. Drawing the ammunition from a quiver, case, or other container is part of the attack (you need a free hand to load a one-handed weapon). At the end of the battle, you can recover half your expended ammunition by taking a minute to search the battlefield.
Sling Bullets are typically stored in a Pouch (bought separately).