David didn’t just take a rock to a sword fight…
David took a sling and some stones, which when accurately used could propel a rock at high velocity toward the target. This weapon actually gave David the advantage because he did not have to feel the brute strength of a giant. This wasn’t the first time David wielded this weapon. David was a shepherd and he protected his sheep from lions and bears. David was not a helpless shepherd boy. He was a strong and fierce shepherd who trusted God and who knew how to defeat beasts, whether they be on four legs or two really large ones.
The Bible says…
David answered Saul, “Your servant has been tending his father’s sheep. Whenever a lion or a bear came and carried off a lamb from the flock, I went after it, struck it down, and rescued the lamb from its mouth. If it reared up against me, I would grab it by its fur, strike it down, and kill it. Your servant has killed lions and bears; this uncircumcised Philistine will be like one of them, for he has defied the armies of the living God.” Then David said, “The Lord who rescued me from the paw of the lion and the paw of the bear will rescue me from the hand of this Philistine.” Saul said to David, “Go, and may the Lord be with you.”
1 Samuel 17:34-37