>Ich habe leider keine Quellenangabe, die belegen könnte, daß der Bindestrich zwischen bull's und eye inkorrekt ist.
that's probably because it isn't wrong
and "not feeling well with the hyphen" !! is no reason to omit it.
The variant without the hyphen as two words could be added, but the hyphen is NOT wrong.
Chambers 21st Dictionary. Copyright Chambers Harrap Publishers Ltd 2007:bull's-eye
noun 1 the small circular centre of a target used in shooting or darts, etc. 2 darts, etc a shot which hits this. [...]http://www.chambersharrap.co.uk/chambers/feat...
The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. bull's-eye
or bull's eye
The small central circle on a target.
A shot that hits this circle.
A direct hit: scored a bull's-eye on the window with a snowball.
The precise accomplishment of a goal or purpose: “With his overflowing style, [he] almost always hits the bull's-eye of universality” (William Zimmer).