in all honesty ... I do not feel well with the hyphen either. :/
Better omit it.

>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
n.
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).
[...]
http://www.answers.com/bull%27s-eye&r=67

Isn't "to hit the bull's-eye" equivalent to "voll ins Schwarze treffen" and "hit the nail on the head" = den Nagel auf den Kopf treffen??

bull's-eye or bull's eye ( ) n.
The small central circle on a target. A shot that hits this circle. A direct hit: scored a bull's-eye on the window.

http://www.answers.com/topic/bull-s-eye?cat=health

s.o.