flak vs ack-ack

flak and ack-ack both are nouns.

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

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

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

flak (noun)	ack-ack (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