Saul armed David - He knew that although the battle was the Lord's, yet prudent means should be used to secure success.

And Saul armed David with his armour,.... Not with what he wore himself; for it cannot be thought he would strip himself of his armour in the field of battle, and when just going to it; and besides what suited the one would not be fit for the other, their bulk and stature being different i but this was some armour Saul had brought with him, besides what he himself wore, to furnish any with that might want it:
and he put an helmet of brass upon his head; such an one, though not so large as Goliath had, these being usually made of brass; See Gill on 1-Samuel 17:5,
also he armed him with a coat of mail; which probably was of brass also, and like that of Goliath's too, only lesser, 1-Samuel 17:5.

Saul armed David with his armour--The ancient Hebrews were particularly attentive to the personal safety of their warriors, and hence Saul equipped the youthful champion with his own defensive accoutrements, which would be of the best style. It is probable that Saul's coat of mail, or corslet, was a loose shirt, otherwise it could not have fitted both a stripling and a man of the colossal stature of the king.

Armour - With armour taken out of his armoury. He seems to speak of some military vestments which were then used in war, and were contrived for defence; such as buff - coats are now.