David was led by The Lord. Don’t try this on your own presumption.
“Reaching into his bag and taking out a stone, he slung it and struck the Philistine on the forehead. The stone sank into his forehead”
Notice the two distinct things that happened here: 1. David slung the stone and it struck Goliath in the head. 2. The stone then proceeded to sink into Goliath’s forehead.
David did his part and then God did his part y pressing the stone with his thumb. He’s stronger than David.