Charles Ellicott Commentary 1 Samuel 17:49

"And David put his hand in his bag, and took thence a stone, and slang it, and smote the Philistine in his forehead; and the stone sank into his forehead, and he fell upon his face to the earth." — 1 Samuel 17:49 (ASV)

And smote the Philistine in the forehead. — The Septuagint adds the words “through the helm.” The Greek translators could not understand the fact of the forehead being unprotected. But the headpieces of the armor then do not appear to have possessed “visors;” the face was covered with the heavy shield, which was carried, we are told (1 Samuel 17:7), before him. No doubt the Philistine, utterly despising his youthful “unarmed” antagonist, advanced toward him without using, as was customary, the face protection of the shield.

Slinging stones had been brought to an extraordinary perfection among the Israelites. Many years before this time, we read that in the tribe of Benjamin there were 700 chosen men left-handed; every one could sling stones at an hair’s breadth, and not miss (Judges 20:16).

A work by W. Vischer, on “Ancient Slings” (Basel, 1866), quoted by Lange, speaks of slingers who could hit the part of the enemy’s face at which they aimed.