How Did David Kill Goliath?

Did David kill the giant, Goliath, with a sling 
and stone or with a sword? From the Topica Discussion list, 8 Feb. 2002:

Re: David and Goliath revisited Feb 08, 2002 

GEISIK
Greetings all.

(NRSV)
1 Samuel 17
49 David put his hand in his bag, took out a stone, slung it, 
and struck the Philistine on his forehead; the stone sank into 
his forehead, and he fell face down on the ground.
50 So David prevailed over the Philistine with a sling and a stone,
striking down the Philistine and killing him; there was no sword in
David's hand.

GEISIK
Pretty clear, right? The standard yarn about David killing him with 
the stone. Well, perhaps Goliath was resurrected because we next 
read:

(NRSV)
1 Samuel 17:51 Then David ran and stood over the Philistine; 
he grasped his sword, drew it out of its sheath, and killed him; 
then he cut off his head with it. When the Philistines saw that 
their champion was dead, they fled.

TILL
The OT is filled with examples of vague pronoun-antecedent 
references, and this could be one of them. The sentence that 
seems to say that David had a sword, which he drew to kill 
Goliath, could have meant this: "He [David] grasped his 
[Goliath's] sword, drew it out of its sheath, and killed him."

If this is what the writer meant, it would still be a mistake in 
my opinion, because I would think that a writer inspired by 
an omniscient, omnipotent deity should have been able to 
write with more clarity than that.

GEISIK
In any case, it does not answer my questions. Exactly how 
did David kill Goliath, stone or sword. 

TILL
Well, you should know that if "dead" didn't always mean dead, 
then "kill" wouldn't always mean kill.

Farrell Till