knocker vs big gun

knocker and big gun both are nouns.

Word	Noun	Adjective	Verb	Adverb
knocker	Yes	No	No	No
big gun	Yes	No	No	No

As nouns, big gun is a hypernym of knocker; that is, big gun is a word with a broader meaning than knocker:

knocker: (Yiddish) a big shot who knows it and acts that way; a boastful immoderate person
big gun: an important influential person

Other hypernyms of knocker include big cheese, big deal, big enchilada, big fish, big shot, big wheel, head honcho.

knocker (noun)	big gun (noun)
a device (usually metal and ornamental) attached by a hinge to a door	an important influential person
either of two soft fleshy milk-secreting glandular organs on the chest of a woman
one who disparages or belittles the worth of something
a person who knocks (as seeking to gain admittance)
(Yiddish) a big shot who knows it and acts that way; a boastful immoderate person