duty tour vs tour of duty

duty tour and tour of duty both are nouns.

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

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

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

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