Ranting, just ranting.

[Rizban]

To be fair to Smaug, the Black Arrow was forged personally by dwarven royalty, designed specifically to slay dragons, may have been magical, and seemed to never break or get lost no matter how many times it was shot.

Bard inherited it from his forefathers, potentially also making the Black Arrow a Legacy Weapon.

Bard also had semi-mystical guidance from the Thrush to direct his shot, rolled a nat 20, and then likely rolled max damage on the crit with his composite longbow.

Edit: If you go by movie rather than book, then it was a bolt fired from a ballista.