ack-ack gun vs flak

ack-ack gun and flak both are nouns.

Word	Noun	Adjective	Verb	Adverb
ack-ack gun	Yes	No	No	No
flak	Yes	No	No	No

As nouns, ack-ack gun and flak are synonyms defined as:

Other synonyms of ack-ack gun include ack-ack, antiaircraft, antiaircraft gun, flack, pom-pom.

ack-ack gun (noun)	flak (noun)
artillery designed to shoot upward at airplanes	artillery designed to shoot upward at airplanes
	intense adverse criticism
	a slick spokesperson who can turn any criticism to the advantage of their employer