USN vs US Navy

USN and US Navy both are nouns.

Word	Noun	Adjective	Verb	Adverb
USN	Yes	No	No	No
US Navy	Yes	No	No	No

As nouns, USN and US Navy are synonyms defined as:

USN and US Navy: the navy of the United States of America; the agency that maintains and trains and equips combat-ready naval forces

Other synonyms of USN include Navy, United States Navy.

USN (noun)	US Navy (noun)
the navy of the United States of America; the agency that maintains and trains and equips combat-ready naval forces	the navy of the United States of America; the agency that maintains and trains and equips combat-ready naval forces