sentry duty vs guard duty

sentry duty and guard duty both are nouns.

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

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

Other synonyms of sentry duty include guard, sentry go.

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