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"According to Bard, this arrow had been passed down to him from his father and grandfather, and he believed it might have originated in the Lonely Mountain. Bard fired the arrow through a bare patch in in the armor on Smaug's underbelly, described to him by the Thrush. The arrow penetrated to Smaugs heart, killing the dragon immediately." I got this from a great site about Tolkien. I'll list it in the source link below.