Hi,
I haven't seen these two studies mentioned anywhere on this site, but I may be mistaking as there is a huge volume of info posted and I didn't managed to go trough all of it yet.
Nevertheless here you have the info:
S. J. Greep, Lead Sling-Shot from Windridge farm, St Albans and the Use of the Sling by the Roman Army in Britain, in Britannia, vol. 18, 1987, pp. 183-200. The article can be also found on Jstor (for those of you who have acces)
http://www.jstor.org/stable/526445R. F. Heizer and I. W. Johnson, A Prehistoric Sling from Loveloch Cave, Nevada, in American Antiquity, vol. 18, no. 2 (Oct 1952), pp. 139-147. Jstor link
http://www.jstor.org/stable/276537I hope you will find them interesting and useful and that I haven't reposted information that was already available.
Cheers!