tour of duty vs duty tour

tour of duty and duty tour both are nouns.

Word	Noun	Adjective	Verb	Adverb
tour of duty	Yes	No	No	No
duty tour	Yes	No	No	No

As nouns, tour of duty and duty tour are synonyms defined as:

Other synonyms of tour of duty include enlistment, hitch, term of enlistment, tour.

tour of duty (noun)	duty tour (noun)
a period of time spent in military service	a period of time spent in military service