ack-ack gun vs antiaircraft gun

ack-ack gun and antiaircraft gun both are nouns.

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

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

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

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

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