The interpretation timeline
1 Samuel 17:49 Douay-Rheims

And he put his hand into his scrip, and took a stone, and cast it with the sling, and fetching it about struck the Philistine in the forehead: and the stone was fixed in his forehead, and he fell on his face upon the earth.

A.D. 397 – 1871 7 witnesses

Patristic

before A.D. 750
397
A.D.
Ambrose of Milan Patristic
A.D. 339–397
“David never waged war unless he was driven to it. Thus prudence was combined in him with fortitude in the battle. For even when about to fight single-handed against Goliath, the enormous giant, he rejected the armor with which he was laden. His strength depended more on his own arm than on the weapons of others. Then, at a distance, to get a stronger throw, with one cast of a stone, he killed his enemy.”
Source
407
A.D.
John Chrysostom Patristic
A.D. 347–407
“Therefore, let us take in our hands that stone, I mean the cornerstone, the spiritual rock. If Paul could think in these terms of the rock in the desert, no one will in any way feel resentment against me if I understand David's stone in the same sense. In the case of the Jews in the desert, it was not the nature of the visible stone but the power of the spiritual stone which sent forth those streams of water. So, too, in David's case, it was not the visible stone but the spiritual stone which sank into the barbarian's head. This is why, at that time, I promised that I would say nothing based on rational arguments. "Our weapons are not merely carnal but spiritual, demolishing sophistries and reasoning and every proud height that raises itself against the knowledge of God."”
Source

Scholastic

c. 1100 – 1500
1105
A.D.
Rashi Jewish
1040–1105
“And he fell face downward. He should have fallen backwards since he had been struck on his forehead. But, in order that Dovid should not be troubled to walk [the extra distance] and cut off his head, for Dovid now saved twelve amos and two zeres; for his height was six amos and a zeres, Dovid saved [walking] his entire height back and hs entire height forward.”
Source

Post-Reformation

c. 1650 – 1900
1771
A.D.
John Gill Reformed
1697–1771
“And David put his hand in his bag,.... The shepherd's scrip, in which he had put the five stones he took out of the brook: and took thence a stone; and put it into his sling he had in his hand: and slang it, and smote the Philistine in his forehead; it is made a difficulty of how he should smite him on his forehead, when he had a helmet of brass upon his head, Sa1 17:5; in answer to this Kimchi observes, that some say, that when David said he would give his flesh to the fowls of the air, at the mention of that he looked upwards, and what was upon his forehead fell backwards, and then David slung and smote him; or he might put back his helmet to talk with David, and hear and be heard the better; and having nothing to fear from an unarmed man, might neglect to put it forward again; or there might be some open space left in the helmet for him to look through, in at which the stone might pass; so the Targum renders it, he smote him in the house of his eyes, so the stone passed through the eye hole into his brain: but after all, supposing his forehead ever so well covered, as the stone slung by David was under a divine direction, so as to hit a person in motion, it came with a divine power, which nothing could resist; and supposing this, of which there need no doubt, it could as easily pass through the helmet of brass, as pierce into his forehead and sink there; nor can this be thought the least incredible, if what Diodorus Siculus (m) relates of the Baleares be true, that they were so dexterous at slinging, that they not only would sling stones bigger than others could, and were so directed, that they seldom missed their mark, being inured to it from their youth, but would even in battle break in pieces shields, helmets, and all kinds of armour, with which bodies were covered: that the stone sunk into his forehead; and so into his brain, as a stone is immersed and sinks in water, when thrown into it; with such force did it go, and with so much ease did it make its way, through the direction and power of God: and he fell upon his face to the earth; Jarchi observes, that it was most natural for him to have fallen backwards, being struck upon his forehead; but so it was, that David might have no trouble to cut off his head, for by this means he fell nearer to him. (m) Bibliothec. l. 5. p. 298.”
Source
1832
A.D.
Adam Clarke Methodist
1762–1832
“Smote the Philistine in his forehead - Except his face, Goliath was everywhere covered over with strong armor. Either he had no beaver to his helmet, or it was lifted up so as to expose his forehead; but it does not appear that the ancient helmets had any covering for the face. The Septuagint however supposes that the stone passed through the helmet, and sank into his forehead: Και διεδυ ὁ λιθος δια της περικεφαλαιας εις το μετωπον αυτου, "and the stone passed through his helmet, and sank into his skull." To some this has appeared perfectly improbable; but we are assured by ancient writers that scarcely any thing could resist the force of the sling. Diodorus Siculus, lib. v., c. 18, p. 287, edit. Bipont, says "The Baleares, in time of war, sling greater stones than any other people, and with such force, that they seem as if projected from a catapult. Διο και κατα τας τειχομαχιας εν ταις προσβολαις τυπτοντες τους προ των επαλξεων εφεστωτας κατατραυματιζουσιν· εν δε ταις παραταξεσι τους τε θυρεους, και τα κρανη, και παν σκεπαστηριον ὁπλον συντριβουσι. Κατα δε την ευστοχιαν οὑτως ακριβεις εισιν, ὡστε κατα το πλειστον μη ἁμαρτανειν του προκειμενου σκοπου Therefore, in assaults made on fortified towns, they grievously wound the besieged; and in battle they break in pieces the shields, helmets, and every species of armor by which the body is defended. And they are such exact marksmen that they scarcely ever miss that at which they aim." The historian accounts for their great accuracy and power in the use of the sling, from this circumstance: Αιτιαι δε τουτων, κ.τ.λ. "They attain to this perfection by frequent exercise from their childhood; for while they are young and under their mother's care, they are obliged to learn to sling; for they fasten bread for a mark at the top of the pole; and till the child hit the bread he must remain fasting; and when he has hit it, the mother gives it to him to eat." - Ibid. I have given these passages at large, because they contain several curious facts, and sufficiently account for the force and accuracy with which David slung his stone at Goliath. We find also in the μη ἁμαρτανειν, not miss the mark, of the historian, the true notion of ἁμαρτανειν, to sin, which I have contended for elsewhere. He who sins, though he aims thereby at his gratification and profit, misses the mark of present and eternal felicity.”
Source
1849
A.D.
1774–1849
“Forehead. “The soul….more probably resides in the callous body of the brain,” (Eyre, Thesis 1797,) between the eyes. (Haydock) — Earth, quite lifeless, (Salien) or unable to resist. (Menochius) — The Balearic slingers scarcely ever missed their mark. (Livy, viii. 4.) The Chaldean supposes that David hit the eye, which was not covered with brass: but the stone might penetrate or kill Goliath through his helmet. Even a buckler is not capable of withstanding their violence. (Diodorus, v. 207.) See Judges xx. 16. (Calmet) — Pride sits on the forehead, and manifests itself by impudent behaviour. We must destroy it by humility, and by the cross of Christ. (St. Augustine) (Worthington)”
Source
1871
A.D.
1871
“smote the Philistine in his forehead--At the opening for the eyes--that was the only exposed part of his body.”
Modern · 1953 →

The in-app commentary runs from the Fathers to the early-modern record, then stops — that's where the public-domain sources end, not where the reading does. For the modern reading, follow the sources directly.