Just wondering what kind of range sling stones or lead sling bullets could attain ? One of my books has a chart that shows range of different missile weapons and a sling using a lead bullet is estimated at being 400 meters. ( Trials with unshaped pebbles reached 260 meters. )

Persian Bow & Rhodian sling about the same at 375 yards.

Longbow 280 - 350 meters.

Large Ballista / onager 300 meters.

Modern World Record spear throw 258 meters 1995 ( Held by David Engwall, using an Atlatl )

Range small Onager 200 meters.

Plumbata 70 meters.

Hand thrown ( Pila, Francisca and stones ) 30 meters.

Source: BARBARIAN WARRIORS, Saxons, Vikings, Normans, by Dan & Susanna Shadrake © Brassey's (UK ) Ltd
Brassey' s History of Uniforms

Anyone know the velocity in feet per second of a lead sling bullet ? A really slow 19th century British large calibre revolver
might be around a lackadaisical 600 ft/sec. Arrows seem to top out at around 200 ft/sec. Anything substantially above 200 ft/sec for slings would be very interesting as gunpowderless bullets !

How long does it take to develop basic accuracy ? The sling projectile at least going in the general direction wanted and being useable as plunging longrange mass fire ? Getting to be as accurate as a decent bowman ?

Ancient world slingers were reputed to be able to hit small targets at fairly long range from what I have read.
The problem is that most of what I have read doesn't quantify at all exactly what the above statistics would be.

As an aside, if I was forming a Medieval army I would think that giving a sling to everyone as at least a backup weapon would be easy as the bulk of the equipment, sling and stones / lead bullets would be small: This assuming at least minimal competence in the use of the sling. Expert slingers being used as specialists and maybe doubling as light infantry and armed with Pollarms and more or less light skirmishers or more heavily armed.

I'm afraid I don't have any numbers for you. However, as one who is still teaching himself to use the sling, I may have a little insight . As far as how long it would take to get people to be able to make the stone go in the right direction, I believe I could teach someone in an afternoon. However, as far as being able to accurately hit small targets, I am still working on that and believe that it would probably take about as long as for a bowman. So in the end, it would seem to have about the same learning curve as a bow, easy to learn how to shoot in the right direction, but years of practice (from youth preferrably) to be really effective.
Hope this helps a little,
Dan

Peter Connolly writes in Greece and Rome at War that slings could easily outrange the composite-bows of the period.
350 meters and more were nothing unusual, especially when lead-shots were used. Xenophon writes how nasty these were - they entered the body and the flesh closed over them, almost the same effect as with early firearms.
Learning to fire accuratley might take quite some time, but especially in mountainous regions like Greece every kid and sheepherder had one (my uneducated guess *g*).