big gun vs big enchilada

big gun and big enchilada both are nouns.

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

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

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

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