U.S.A. vs billionth

U.S.A. and billionth both are nouns.

Word	Noun	Adjective	Verb	Adverb
U.S.A.	Yes	No	No	No
billionth	Yes	Yes	No	No

U.S.A. (noun)	billionth (noun)
North American republic containing 50 states - 48 conterminous states in North America plus Alaska in northwest North America and the Hawaiian Islands in the Pacific Ocean; achieved independence in 1776	one part in a billion equal parts
	position 1,000,000,000 in a countable series of things

U.S.A. (adjective)	billionth (adjective)
	the ordinal number of one billion in counting order