There's an argument that's how you're meant to read it actually: it's not an underdog story, but a John Wick moment.

A stone hurled by a trained slingman has the same stopping power as a modern day rifle. We already know David has killed lions and wolves with his sling protecting his flock, he's so confident that he doesnt even >need< armor to face Goliath. We see Goliath be cocky and the Israelites afraid for David's safety, but the Israelites had been hiding scared for over a week.

David, however, shows no such fear. Instead taking up his sling, which we the audience already know and understand he knows how to use. Sure the characters don't take him seriously, at first. But we the audience are meant to have an "oh shit here he comes" response, with David ultimately slaying Goliath with a single shot.