This is a subject I know little about but find fascinating.  Is there any way you can share these photographs with us?

David, You're entirely correct that throwing from an elevation will increase the range of a sling. In surveying hillforts, at what distance from them will you generally begin to find slingstones? After the weather breaks over here I fully intend on running tests.  
 

Quote from kentuckythrower on Jan 30^th, 2012, 9:25am:

 
Conversely, slinging uphill reduces the range considerably. Assuming both the extension and reduction in range to be around 50 m, would mean that attackers would have to cross up to 100m under fire before getting close enough to return it.(?)
 
Considering that the stones thrown from ancient hilltop forts would have been radially dispersed over a large area, would it be possible to identify a stone as having been thrown from it with any degree of certainty?

Quote from Aussie on Jan 30^th, 2012, 3:49pm:

 
At Maiden hill, the ammo dumps were made up of large, ideally sized and shaped stones from a river bed some ways off from the main site, so yes, they would be recognizable. But generally, I don't think Maiden Hill or the other Iron Age hillforts have produced this sort of archaeological record (dispersion of shot). There is one example where you can trace very precisely the distribution of shot (lead) in battle condition:
 
A. V. A. J. Bosman, Pouring Lead in the Pouring Rain:
Making Lead Slingshot under Battle Conditions, dans: C. van Driel-Murray (éd.), Roman
Military Equipment: Experiment and Reality, JRMES 6, 1995, 99–103.
 
And at Maiden Hill, the Romans used catapults (sharp-throwers) to target the defenders-- you can actually trace ranging shots, and one particularly good gunner, from the pattern of distribution of the bolt heads in the chalky soil on the hillfort.  

Kentuckythrower (and anyone):  
If you want details and have access to a library with archaeological sources, I suggest looking at the following references. In terms of where slingstones are found, the evidence is all from pits filled with them (presumed to be for use in defence). There were thousands in some cases. Finney discusses sizes etc (I think his monograph is mentioned elsewhere on this site).  
 
Pete
 
Avery, M. 1993a. Hillfort defences of southern Britain: Vol. 1 - Ramparts, entrances, dating, prehistory. BAR British Series 231. Oxford: British Archaeological Reports.
Finney, J. B. 2006. Middle Iron Age Warfare of the Hillfort Dominated Zone c. 400 BC to c. 150 BC. BAR British Series 423, Oxford: British Archaeological Reports.
Wheeler, R. E. M. 1943. Maiden Castle, Dorset. Oxford: Research Report of the Committee of the Society of Antiquaries 12.
 
and for everything you ever wanted to know about the British Iron Age, but were afraid to ask:
Cunliffe, B. W. 2005. Iron Age Communities in Britain. 4th (paperback) ed. Abingdon: Routledge.

Hi Pete,
I'll see if my university's library has these references. The problem I generally face over here is it seems all the attention is being paid to either New World or Classical Archaeology. There seems to be a dearth of anything related to the Old World outide the Roman, Greek, or Egytian references.  

Hi Mauro,
Glad to make contact with you. I have got to tell you the lack of funding seems to be prevalent worldwide. The bulk of my studies are oriented towards New World Archaeology, and The Old World Archaeology is just skimmed over. I find this unacceptable since my primary interest is in the prehistory of Europe.
 
Getting back to slinging and helping Pete in his studies. This topic has really grabbed my interest as it seems to hit home...I am of British ancestry. I'd like to re-discover my ancestry and the way they survived. The attack and defense of the hillforts falls right in line with my background. With this in mind, I have taken on this project for very personal reasons.  
 
I don't know if you have read too much about me, but I've been using slings for better than four decades now and have quite a bit of experience in how they function. Despite the fact I've not been formally "trained" in their use, I can definitely put a stone on target. What I want to do is accurately replicate slinging to gain a better understanding of what it must have been like to actually attack, or defend one of these hillforts.  
Dave