Hondero, you got me curious enough to go back and read the passage in an old Loeb Classical Library edition of Livy.  Here's what precedes the description of the cestrosphendone:

"...he withdrew the soldiers, who were dismayed by the sudden arrival of the enemy, to a nearby hill, intending to defend himself by the strength of the position since he was no match in numbers and power.  There when he had gathered his soldiers in a circle, so that with their close-packed shields they could protect themselves from the blows of arrows and javelins, Perseus, with his men-at-arms surrounding the hill, ordered some to try the ascent from all sides and join battle at close quarters, others to hurl weapons from a distance.  A terrific threat surrounded the Romans, for when massed they could not thrust back those who were struggling up the hill, and whenever they broke ranks by charging forward, they were exposed to javelins and arrows."

The next line--introducing the description of the cestrosphendone--reads:  "They suffered particularly from the dart-slings."

I related this to their entire predicament,  the "terrific threat" that surrounded them as described in the preceding sentence.  When they were bunched up, they would have been vulnerable to darts raining down on them from a high trajectory, but when they advanced they opened themselves up to flatter trajectory arrows and javelins.  Because the "suffered particularly from the dart-slings" follows upon "they were exposed to arrows and javelins," I can see how others might interpret the dart-sling as a flatter trajectory weapon seemingly included with the arrows and javelins.      

Hondero, your English as a second language is better than a good many people's English as a first language.  When I first read the Livy passage years ago, my subjective interpretation of how such damage could be done was that the darts were raining from a high trajectory into a densely packed formation.  This was probably because my own slinging as a young man was exclusively underhand distance slinging, trying to hit boulders on the other side of a wide river.  The gravity-aided force with which the stones came down from their high trajectories made quite an impression on me of the damage that could be done to a human target.  Now imagine finned spikes coming down from a high trajectory into a closely-packed formation of men. 

You are quite right that the text doesn't specifically point out that darts rained down into the formation on the hilltop.  However, I find it curious that the darts weren't mentioned in the two "arrows and javelins" phrases in the passage.  The text states that the Romans circled up with raised shields to protect themselves against "arrows and javelins," not "arrows, javelins, and darts."  When they broke ranks and charged forward, they were exposed to "javelins and arrows," not "javelins, arrows, and darts."  If the darts had been launched in unison with the arrows and javelins, it seems the text would have read something along the lines of "when they charged forward, they were exposed to javelins, arrows, and darts.  They suffered particularly from the darts..."

Darn!  I've got to go to work now.  I'll share more on this fascinating subject later.            

DuckofDeath, I´m out of home this week, even without a dictionary (I feel lost) but I promise you to answer your questions soon. The weight of the cestros point can be calculated easily, in fact I have some bolts of similar size. If you can translate latin text of Livy, is interesting the exact meaning of "he gathered his soldiers in a circle, so that with their close-packed shields.." because there are two ways of packing the shields: frontal around like a circular wall or over the heads like a ceiling, and so to know if the cestros were thrown like a massive rain or like accuracy shots
I´ll be here soon...

Thanks. I've been looking for a Latin-English dictionary forever....

Some random cogitations on Livy...

The text states that the Romans made a circle to protect themselves against the blows of arrows and javelins, indicating that perhaps as they were being surrounded by the Macedonians they were already being assailed by those weapons.  (The translation seems to match the Latin text fairly well.) Now here's the interesting part:  After the Romans had their shields up to protect against arrows and javelins, Perseus orders some of his men to attack and others to "hurl weapons from a distance."  The infinitive translated "hurl" is ingerere, and one of its meanings is "to throw into or upon a place," perhaps suggesting concentrated fire into or upon the packed Roman formation.  The word "weapons" is tela, from telum, literally "missile weapon"; the
cestrosphendone is subsequently described as a teli, a form of telum.  It would seem that other tela were brought into play after the initial attacks by bow and javelin.

     

that's really interesting

well i will be taking latin when school starts back up here so maybe i will be of more help then.  or perhaps i can get my tacher to take a stab at it

I've done Greek GCSE, so I could take a stab, but don't expect great things.