I like the sling and I'm sorry to play again the bad boy but I have great doubts about the effectiveness of slings against armoured opponents.
First I would like to link this, it contains a few considerations of mine:
http://www.romanarmy.com/rat/viewtopic. ... c&start=40
Sling bullets travel at a very low speed. The speed is roughly the same as that of arrows. I doubt that any significant heating can be achieved at velocities of about 50 m/s to 70 m/s; if so it has nothing to do with the weapons performance as hot bullets are no better penetrators. As blunt trauma effects are mentioned: modern aramid vests are also effective without shock absorbing devices. This against bullets which travel with velocities up to 400 m/s and have energies of 500 to 700 Joule. The hit may hurt like hell (if a person in a fight, full of adrenaline, feels the pain) but permanent injuries are the exception. Reportedly (I am not so comfortable with the source, the book "From Sumer to Rome") the US Army assumes an energy of about 800 Joule necessary to wound a soldier through the US kevlar helmet. The ancient helmets were of course far worse in performance but faced also only low energy threats.
The energy and momentum of sling bullets equals more or less that of arrows and javelins. The energy is so low (50 to 150 Joule, depending on the authors thoughts about sling bullet velocity) that severe wounds through armour are not to be expected. That a glandes could crash a scutum is highly unlikely; every javelin would have done the same then.
Except some more or less peculiar and anectodic reports from the ancient times (like that of the hot bullets penetration) I know only one incident when slingers seemed to be decisive against armour, at the battle of Eknomos 311 BC. The Balearic slingers are said to have used very heavy stones (a mnai weight), so perhaps they were really able to crush some aspides and helmets.
I have no data at hand but I remember that when numbers were given in the sources often the slingers were less frequent than archers. "Thousands of slingers" in any case are highly unlikely (at Eknomos 1000 Balearic slinger are mentioned afaik). May it be just for the reason that they would need a huge space to be deployed.
Of course sling bullets were effective against unarmoured parts of the body but were not a secret mass destruction weapon against armoured and/or shielded heavy infantry.