I am not sure how a regular sling would go against a helmet but I would be interested if someone knows. Still this suggests that armor does work.

http://www.youtube.com/watch?v=4FCxwC-97YQ

As far as slinging goes, if you use a big enough rock, the helmet may save his skull but the impact will break his neck.

Bill Skinner wrote on Nov 17^th, 2012 at 10:48am:

As far as slinging goes, if you use a big enough rock, the helmet may save his skull but the impact will break his neck.

Only Joerg (or King Kong) could shoot a strong enough slingshot for that.

In general it does seem that armor does work... that is the whole point of going through all that trouble of expense and encumberance... but no armor is perfect. Yes, a good helmet does shut down most instakill sling shots, but a helmet can be worn down, one can still get a concussion through a helmt, it is true a neck can be broken, an opponent can still be knocked down (or off a horse)... all dependent on the nature of the particular helmet of course. But here's the other thing... helmets only protect so much... what about the face? The neck? and oh I don't know... the rest of the body and limbs (especially joints). Also for most of history the majority of soldiers have no armor at all including helmets... and many helmets through history are not of the utmost quality... brass and bronze often bending in anyway (depending on thickness) and iron either having the same problem or flat out cracking and breaking after a sling hit... better suited to deflecting the points of spears and arrows... and better at resisting the lesser impact of hand thrown stones.

Receiving a 4 oz stone from 200m unto your helmet will save you ,but IMO you could miss the battle laying unconscious, or be seriously dizzy, or just being very disoriented for 10min- 1 hour.  which would seriously lower your survival chances in melee combat.
Of course it could be a glancing shot,without much problem,maybe the guy behind you would miss 5-6 teeth.

The example Fundibularius quotes (from Caesar's Gallic campaign) has a Roman commander hit in the mouth by a slingstone. I think Paullus was hit in the head by a slingstone at Cannae; at Thermopylai, Antiochos III was hit in the mouth by a stone-- hand or sling ? (and lost teeth-- he had already lost some in a cavalry battle in Baktria). Literary sources, but perhaps show that a long-range, depleted-energy stone hit to the head is survivable.

Though I would imagine most of those hits would have taken them out of the fight or seriously impaired their ability to fight on. A shot to the mouth in particular is interesting... unlikely to kill you, but lips are very sensitive and blood rich and losing teeth can mess with you. Even today with modern firearms shooters try to avoid the mouth as teeth (being one of the hardest biological substances) will often shatter while deflecting the bullet. Similarly a sling bullet or stone to the mouth is also likely to knock out or destroy teeth but not kill you.