John Gill Commentary 1 Samuel 17:50

"So David prevailed over the Philistine with a sling and with a stone, and smote the Philistine, and slew him; but there was no sword in the hand of David." — 1 Samuel 17:50 (ASV)

So David prevailed over the Philistine with a sling and with
a stone
And with them only, without any other warlike weapon:

and smote the Philistine, and slew him ;
smote him with the stone, which brought him to the ground, and then slew him with his own sword, as afterwards related:

but [there was] no sword in the hand of David ;
when he engaged with the Philistine, and smote him, for he had put off all his armour, (1 Samuel 17:39) .