Matthew Poole and continuators on 1 Samuel 17:40

40 And he took his staff in his hand, and chose him five smooth stones out of the brook, and put them in a shepherds bag which he had even in a scrip, and his sling was in his hand , and he drew near to the Philistine.

His Shepherds 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.

That if one should fail him, he might make use of another.

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.

This sort of Weapons was not unusual in the Fights of ancient times; and many arrived at great dexterity in slinging Stones with great certainty; of which we have instances both in Scripture, as Iudg. 20. 16. and in Diodorus Siculus, and Livy, and other Authors.

Marginal references: Or, Valley.; Heb. Vessel.