Sunni vs Sunni Muslim

Sunni and Sunni Muslim both are nouns.

Word	Noun	Adjective	Verb	Adverb
Sunni	Yes	No	No	No
Sunni Muslim	Yes	No	No	No

As nouns, Sunni and Sunni Muslim are synonyms defined as:

Sunni and Sunni Muslim: a member of the branch of Islam that accepts the first four caliphs as rightful successors to Muhammad

Other synonyms of Sunni include Sunnite.

Sunni (noun)	Sunni Muslim (noun)
one of the two main branches of orthodox Islam	a member of the branch of Islam that accepts the first four caliphs as rightful successors to Muhammad
a member of the branch of Islam that accepts the first four caliphs as rightful successors to Muhammad