Answer: Well Bard's previous attempts at wounding Smaug have failed, but a thrush from the mountains of Erebor has revealed to him that Bilbo has discovered an unheard source of weakness, his missing scale beneath his left breast. Then he fires his last shot that finds a gap in Smaug's scales, killing him and sending him down the river. Furthermore, JRR Tolkien decided that Gandalf did not kill Smaug with the intention of becoming useful to Sauron.
Explanation: Hope it helps =)
