1 Samuel 17:40

His staff; his shepherd's staff. These arms were in themselves contemptible, yet chosen by David; partly, because he had no skill to use other arms; partly, because he had inward assurance of the victory, even by these weapons; and partly, because such a conquest would be most honourable to God, and most shameful and discouraging to the Philistines. Chose him five smooth stones, that if one should fail him, he might make use of another. Smooth stones, because such stones would go most freely out of the sling; and consequently, with more force and certainty, directly to the mark which he aimed at. The sling was a sort of weapon not unusual in the fights of ancient times, and many arrived at great dexterity of slinging stones with great certainty; of which we have instances both in Scripture, as Judges 20:16, and in Diodorus Siculus, and Livy, and other authors.