big enchilada vs big gun

big enchilada and big gun both are nouns.

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

As nouns, big enchilada and big gun are synonyms defined as:

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

big enchilada (noun)	big gun (noun)
an important influential person	an important influential person