Sling bullets are missiles that are used in conjunction with standard sling-type missile weapons - such as sling, sling of accuracy, sling of the long shot or Whirlwind. Lead sling bullets are roughly comparable to rocks, and higher metal ones are obviously better. However they are rare and not really milkable. In 1.2.x and later, sling bullets can be of slaying types, they are covered in Sling bullet of *foo* slaying topic.