I don't see it this way. The text says; verse 50, "he struck the Philistine and killed him; but there was no sword in David's hand."
Read on, verse 51. The cutting off of the head was to convince the army of the Philistines that their champion was dead.
The stone killed him, the decapitation proved it to his army.
We’ll have to agree to disagree. Since the word slay did not mean “to kill”, but rather “to strike” at the time that that word was used in translating the Bible, it’s unacertainable from the text provided. In fact, even from the original hebrew there is some debate because the word for “forehead” is akin to another word (IIRC the leg, or greaves, something like that), implying that the stone caused Goliath to fall, but was not dead (again, the word “slay” didn’t mean “kill”). Also probable is the scenario that another FReeper mentioned, in that the wound may have been mortal, but Goliath was not necessarilly “fully” dead. In fact there are multiple stages of death.
In any event, I think as someone else posted, it probably doesn’t matter. Having said that, God answers all prayers. The best answer one can receive is the one received by the Holy Spirit after prayerful study and meditation, and I’ll leave that as a personal issue.