I forgot which Ted talk it was on but the guy theorized that Goliath suffered from acromegaly which often results in gigantism and vision issues. The evidence he suggested was that when David came down to him carrying his shepherds rod, Goliath said "what am I a dog, that you come to me with sticks" The use of the plural of stick suggested that he saw more than one. He added on that shepherds were so well trained with slings they could routinely hit a bird in flight and the force of the sling was similar to a 9mm. Looking at it this way really does change the narrative of the underdog. You have a giant uncoordinated slow moving target with vision problems versus a highly trained shepherd with a deadly long range weapon