Longshot is a direct upgrade to the starter bow.
As the name suggests, it has vastly increased range capabilities compared to the original bow. The same arrows can be fired between both bows.
Longshot can be found at the shortcut to Floor 11, at the base of the tower concerned with the collection of the lantern-bits. It will be at the top of the lift, next to a chest. The entrance to the shortcut can be found on the left side of the
scalable wall leading to the floor 13 drop-off, after fighting a horde of enemies appearing from the center of a dangerous room in the Necropolis zone.