open sight vs gun-sight

open sight and gun-sight both are nouns.

Word	Noun	Adjective	Verb	Adverb
open sight	Yes	No	No	No
gun-sight	Yes	No	No	No

As nouns, gun-sight is a hypernym of open sight; that is, gun-sight is a word with a broader meaning than open sight:

open sight: rear gunsight having an open notch instead of a peephole or telescope
gun-sight: a sight used for aiming a gun

Other hypernyms of open sight include gunsight.

open sight (noun)	gun-sight (noun)
rear gunsight having an open notch instead of a peephole or telescope	a sight used for aiming a gun