antiaircraft vs ack-ack gun

antiaircraft and ack-ack gun both are nouns.

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

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

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

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

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

antiaircraft (adjective)	ack-ack gun (adjective)
designed for defense from a surface position against air attack