guard duty vs sentry duty

guard duty and sentry duty both are nouns.

Word	Noun	Adjective	Verb	Adverb
guard duty	Yes	No	No	No
sentry duty	Yes	No	No	No

As nouns, guard duty and sentry duty are synonyms defined as:

Other synonyms of guard duty include guard, sentry go.

guard duty (noun)	sentry duty (noun)
the duty of serving as a sentry	the duty of serving as a sentry