Again, you're free to believe what you like. If you want to believe that a stone from a young boy's sling crushed the skull of a giant, that's entirely your prerogative. Personally, I believe that David confronted Goliath because he knew God was with him, and even though he was out-manned physically, God made the impossible possible. Note what David said he was coming against Goliath with (emphasis added);
1 Samuel 17:45 (NIV)
David said to the Philistine, “You come against me with sword and spear and javelin, but I come against you in the name of the Lord Almighty, the God of the armies of Israel, whom you have defied.
Notice that David didn't say, "I come against you with a sling and a stone that will crush your skull!" David had faith in God, not in his weapon.