buck private vs common soldier

buck private and common soldier both are nouns.

Word	Noun	Adjective	Verb	Adverb
buck private	Yes	No	No	No
common soldier	Yes	No	No	No

As nouns, buck private and common soldier are synonyms defined as:

buck private and common soldier: an enlisted man of the lowest rank in the Army or Marines

Other synonyms of buck private include private.

buck private (noun)	common soldier (noun)
an enlisted man of the lowest rank in the Army or Marines	an enlisted man of the lowest rank in the Army or Marines