Why not look at the text? (1 Sam 17, 40 ff):

40 He took his stick in his hand, selected five smooth stones from the river bed and put them in his shepherd's bag, in his pouch; then, sling in hand, he walked towards the Philistine. 
[...]
48 No sooner had the Philistine started forward to confront David than David darted out of the lines and ran to meet the Philistine. 
49 Putting his hand in his bag, he took out a stone, slung it and struck the Philistine on the forehead; the stone penetrated his forehead and he fell face downwards on the ground. 
50 Thus David triumphed over the Philistine with a sling and a stone; he hit the Philistine and killed him, though he had no sword in his hand. 
51 David ran and stood over the Philistine, seized his sword, pulled it from the scabbard, despatched him and cut off his head. When the Philistines saw that their champion was dead, they fled. 

Goliath certainly could have been knocked out by a sling. If he was laying there, no moving, he would seem dead.

Aussie wrote on Nov 1^st, 2009 at 5:32pm:

The discussion I mentioned rested on the argument that Hebrew words for "forehead" and "knee" are closely related, and on, if I understand the paper (not certain!) a change in the inflection of the noun between an earlier text and later ones. Also that in most (though not all) cases the armor of the Philistines is shown as entirely covering the forehead.

The author of the paper remarks that the currently held translation meaning "forehead" was puzzled over by Jewish scholars in Medieval times.

Aussie wrote on Nov 1^st, 2009 at 5:32pm:


Furthermore, David had promised to do so in the rhetorical duel in the passage I had left out above:

41 The Philistine, preceded by his shield-bearer, came nearer and nearer to David. 
42 When the Philistine looked David up and down, what he saw filled him with scorn, because David was only a lad, with ruddy cheeks and an attractive appearance. 
43 The Philistine said to David, 'Am I a dog for you to come after me with sticks?' And the Philistine cursed David by his gods. 
44 The Philistine said to David, 'Come over here and I will give your flesh to the birds of the air and the wild beasts!' 
45 David retorted to the Philistine, 'You come to me with sword, spear and scimitar, but I come to you in the name of Yahweh Sabaoth, God of the armies of Israel, whom you have challenged. 
46 Today, Yahweh will deliver you into my hand; I shall kill you, I shall cut off your head; today, I shall give your corpse and the corpses of the Philistine army to the birds of the air and the wild beasts, so that the whole world may know that there is a God in Israel,  47 and this whole assembly know that Yahweh does not give victory by means of sword and spear -- for Yahweh is lord of the battle and he will deliver you into our power.' 

It was surely Goliath´s sword wich was used for the decapitation, as David went into the duel only with his staff and his sling. This is stressed several times: once in verse 40, then in verse 43, and, before the whole description, in these verses:

38 Saul dressed David in his own armour; he put a bronze helmet on his head, dressed him in a breastplate 
39 and buckled his own sword over David's armour. David tried to walk but, not being used to them, said to Saul, 'I cannot walk in these; I am not used to them.' So they took them off again. 

It seems that to the question: Did the battle between David and Goliath really happened?  there´s just no way of knowing.
But to the question: Can a slingshot to the head kill a really big man in armor? All of us who have been slinging for a while, know that the answer is yes.

It´s unlikely we´ll reach consensus to whether the event happened or not. Killing a man with a sling, for sure.

Speaking of which, anyone knows the distance of the opponents at the begibning of a traditional duel like this?
The Baleric slinger demo of D&G was a 15 meter shot. Does anyone know this for a fact? Or is it an educated guess?

Remember that after the shouting match Goliath begins to approach David, then David runs toward Goliath, and then suddenly use his sling. 
timann

"...and the Stone Sank into His Forehead". A Note on 1 Samuel XVII 49
Ariella Deem
Vetus Testamentum, Vol. 28, Fasc. 3 (Jul., 1978), pp. 349-351

-- argues that the word is "greave" and not forehead