User blog comment:WitchKingofMorgul/Black Arrow theory/@comment-3981209-20130203210119/@comment-3981209-20130203215537

Why would the dwarves have used such an unfamiliar metal as a gift when their tried and tested metals, such as mithril, would serve better? They would use it for themselves first, and not gift it. If it worked well, they wouldn't give away something with a finite supply of metal. If it didn't work well, I suppose Thror was too honourable to pass on rubbish metal gear as a gift to a close friend, and it wouldn't have pierced Smaug's hide. I think it was simply a sharper arrow, at Smaug's weakest point, full force. Longbows are strong enough to punch through plate armour, it makes sense it could punch through scales.