ack-ack vs ack-ack gun

ack-ack and ack-ack gun both are nouns.

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

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

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

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

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