Fanaelialae
Legend
If it was a magical arrow that always landed a kill shot, then why did Bard save it for last? Was he some kind of psycho who enjoyed seeing how high a body count Smaug could rack up? He certainly doesn't across as such in the book. It seems it was his trump card that he was saving, and had the Thrush not told him where Smaug's weak spot was, it seems unlikely it would have worked. Otherwise, why even include the Thrush in the book? Tolkien could have written that Bard blindly fired the arrow and like a heart seeking missile it just found its mark. That's not how he wrote it though.I didn't miss it. The text in the story that I quoted specifically calls out how the arrow never misses. That seems to be pretty significant. More than just a simple magic item. That is, that arrow is what allowed him to make the shot where no other arrow would have been possible, called shot or no.
I don't contest that the arrow likely helped him make the shot. However, if he had handed it to one of the fishermen and asked them to take the shot, I don't think things would have turned out as well. Give a high level fighter a reusable +5 arrow and I think you'd find that they consider it pretty darn infallible and make every effort to recover it too.