antiaircraft gun vs ack-ack

antiaircraft gun and ack-ack both are nouns.

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

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

antiaircraft gun and ack-ack: artillery designed to shoot upward at airplanes

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

antiaircraft gun (noun)	ack-ack (noun)
artillery designed to shoot upward at airplanes	artillery designed to shoot upward at airplanes