The Droid Assassin's rifle is essentially a greatly altered disruptor rifle, and it saw its main usage with the HK-series assassin droid during the time of the Old Sith Wars. This weapon received upgrades such as better sights, less recoil, greater distance, and multiple available slots to add even more modifications.
Within the game Star Wars: Knights of the Old Republic II: The Sith Lords, the sole character capable of equipping the weapon is HK-47.