Matthew Poole and continuators on 1 Samuel 17:49

49 And David put his hand in his bag, and took thence a stone, and slang it, and smote the Philistine in his fore-head, that the stone sunk into his fore-head , and he fell upon his face to the earth.

Either, 1. The Stone pierced through his Helmet; which such Stones being slung, would not seldom do; as even Diodorus Siculus relates. Or 2. The Stone might get in through one of those holes which are left in Helmets, that he that wears it may see his way, and how to direct his blows. Or rather, 3. The proud Giant had lift up that part of his Helmet which covered his Fore-head; and that in contempt of David and his Weapons, and by the singular direction of Gods Providence.