I did post a while back on early weaponry, including that of the sling. In the latest Current Archaeology is an article on Burnswark, Scotland, where the Romans assaulted a hill fort using such primitive but effective weapons. Three types of lead bullets were found there: shaped as a lemon, as an acorn, and a smaller type with a hole in it.
We undertook a ballistic assessment of the lead sling-bullets previously found on the site in order to assess the performance of this ammunition. A number of replica bullets were cast in lead and high-density clay, and one of our volunteers, an expert slinger, learned towage slings from various materials. We found that the larger 50g bullets could be cast at least 200metres depending on whether the low-trajectory direct (more accurate) or lob style (greater distance) of slinging was used. Other experimenters in the fields have noted that a 50g Roman bullet propelled from a sling has only slightly less kinetic energy than a shot from a .44 Magnum! …
Two extraordinary facts concerning the small bullets with holes … also emerged. First, they could be successfully slung in small groups of three or four to create a form of grapeshot. This had been independently confirmed by T. Richardson in his work on Roman sling-bullets at the Royal Armouries. Even more intriguingly, the mysterious holes proved to confer an aeroponic quality: in flight, these lead shot whistled, or more accurately, gave off a mechanical buzzing sound eerily reminiscent of an agitated wasp. Remarkable as it sounds, the simplest explanation for this design modification is that it represents an early form of psychological warfare. To put it another way, the rom,an attacker value date terror that hearing the incoming bullets would install in the defenders.
You can hear this at http://www.archaeology.co.uk/articles/features/burnswark.htm