Quote from kuggur slingdog on Jul 31^st, 2010, 10:30am:

 
 
No I don't think you can see an arrow on a flat trajectory any more then you can see a glans. Roman reenactment group was mentioned in Atlums post. While I am not saying the arrows were not released on a flat trajectory, most of your arrows and sling bullets would be released in the first missle phase which is at close to maximux effective range.
 

I cannot vouch for not seeing sling stones while flying at me, but I can say personally that it is possible to see incoming arrows being volleyed or shot at you directly. I reckon though personally slings are just as tough to see as golf balls and I dont know if people on the green can see incoming golf balls, but that would be the closest comparisson I could think.
Another point though is that if they shoot with the sun to their backs, it is near impossible to see an arrow coming at you.
 
Nemo

Its easy to see a sling stone when you know where its going and pick it up early in its flight while its still close to you.  
 
If somebody else slings and you dont see it leave the pouch then you have very little chance of finding it in the air mid-flight.  
 
If faced with professional slingers shooting from 200+ yards away with lead ammo, you will hear the bullets flying and hitting but you will probably see nothing at all.

Guys I managed to track dow the website for the reenactment gruop. I forgot I had their buiseness card.
 
http://home.surewest.net/fifi/index9.htm
 
They also have a page of typical Roman battle tactics including the one from my previous post.
 
http://www.home.surewest.net/fifi/index20.htm
 

Quote from Thearos on Aug 8^th, 2010, 10:51am:

 
I think you read it wrong. It says that the Archers and Slingers advance 25 yds ahead of the legionaries. Putting them at about 75 yds from the enemy.

Hey I will be seeing the reenactment group this saturday at the Highland games in Pleasanton, CA. I can just ask them the question in person.

I think (but could be proved wrong) that there is no evidence for full-time, specialized slinger auxiliary units in the imperial army (say Augustus to the Severans, C1st BC to AD 250)-- auxiliaries are all-rounder infantry, cavalry, and archers. So slingers in the imperial army are either local levies, allies or mercenaries (the barbarians on Trajan's column), legionaries wih special skills, or auxiliaries on special duties. The slingers in Germanicus' campaigns might be local levies; the "libritores", if they are staff-slingers, might be legionaries or auxiliaries on light duty.  
 
Earlier, during the Republic ? Slingers might be in allied contingents levied in Italy; light skirmishers in the "velites" who chose to fight with sling rather than the usual shield and javelins; specialized contingents, like the slingers sent by the Aitolians to fight at Numantia or by the Attalid kingdom to fight e.g. in the Third Mac. war or in the Achaian War. Caesar in Gaul has a bunch of Baleares, I think-- surely mercenaries.  
 
Conclusion: "the Roman army" at no time had full-time slingers, even when it has full-time soldiers (starting, efectively, in the late Republic); but specific Roman armies have slingers in their ranks, levied, hired, or summoned; ROman soldiers also knew how to sling, and did.  Never huge numbers-- hundreds, a few thousands at most.

I resume this post because it has stimulated my archaeological passion.
I have even resumed my university books and looked in them for an answer.
I will only speak about my region's contribute to the power of Rome.
 
And I've found this:
from "Archeologia nelle Marche" (Luni, Banca delle Marche, 2000):
"I Romani... strinsero un trattato con i Piceni nel 299 a.C., nel quale 360'000 Piceni ottennero la fiducia dei Romani (Plinio)". I translate:"Romans made a deal with Piceni in 299 b.C., that 360'000 of them become loyal to Rome". Romans needed a helping hand against Celts, and accorded loyalty to Piceni, the ancient inhabitants of my Region. It is known that Piceni slingers effectively helped the Roman Army in the Trasimeno and Cannae Battles.  
from "Storia Romana" (Geraci-Marcone, Le Monnier Università, 2000):
"Gli alleati di Roma... erano obbligati a fornire contingenti di fanti pari a quelli dei legionari, per un totale di due legioni di 4.200/5.000 fanti ciascuna...". It means:"Roman's allies had to give a number of foot soldiers equal to two legions of 4'200-5'000 soldiers both".
 
If we consider that, at the beginning of the IIIrd century b.C., there where 360'000 Piceni in my region, and that in the Cannae battle (216 b.C.) about 50'000 Roman soldiers were killed (out of 80'000), here's that Picen soldiers were about 10'000; I suppose that slingers must have been less of that number, but not too much because Piceni were renowed for their ability with this kind of weapon.
Hope to have helped a bit!
Greetings,
Mauro.