guard duty vs sentry go

guard duty and sentry go both are nouns.

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

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

Other synonyms of guard duty include guard, sentry duty.

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