So David prevailed over the Philistine with a sling and with a stone, and struck the Philistine, and killed him; but there was no sword in the hand of David.
After that smooth stone hurled from David's sling "sank into the forehead" of Goliath, he was as dead as if he had been shot between the eyes with a deer rifle!  Subsequent references to David's "killing" Goliath are merely repetitions for the sake of emphasis.