The Hidden Gun was a weapon used by the Animi Training Program's Amimi Avatars and The Order of Assassins. It consisted of a gun attached to a bracer, which was then worn on the wrist. It could eliminate the contract target from a distance, though it took some time to aim and it gave you a reduced score.
The hidden gun was based off of the Assassin Altaïr Ibn-La'Ahad's Codex.