David defeated the Philistine with a sling and a stone. Even though David had no sword, he struck down the Philistine and killed him.
1 Samuel 17:50 In-Context
As the Philistine moved closer to attack him, David ran quickly toward the battle line to meet him.
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, and he fell facedown on the ground.
So David triumphed over the Philistine with a sling and a stone; without a sword in his hand he struck down the Philistine and killed him.
David ran and stood over him. He took hold of the Philistine’s sword and drew it from the sheath. After he killed him, he cut off his head with the sword. When the Philistines saw that their hero was dead, they turned and ran.
Then the men of Israel and Judah surged forward with a shout and pursued the Philistines to the entrance of Gath and to the gates of Ekron. Their dead were strewn along the Shaaraim road to Gath and Ekron.