This Bible contradiction is from the Skeptic's Annotated Bible.
With a sling.
1 Samuel 17:49-50 View context
And David put his hand in his bag, and took thence a stone, and slang [it], and smote the Philistine in his forehead, that the stone sunk into his forehead; and he fell upon his face to the earth.
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.
With a sword.
1 Samuel 17:51 View context
Therefore David ran, and stood upon the Philistine, and took his sword, and drew it out of the sheath thereof, and slew him, and cut off his head therewith. And when the Philistines saw their champion was dead, they fled.