bullet vs rubber bullet

bullet and rubber bullet both are nouns.

Word	Noun	Adjective	Verb	Adverb
bullet	Yes	No	No	No
rubber bullet	Yes	No	No	No

As nouns, rubber bullet is a hyponym of bullet; that is, rubber bullet is a word with a more specific, narrower meaning than bullet:

bullet: a projectile that is fired from a gun
rubber bullet: a bullet made of hard rubber; designed for use in crowd control

Other hyponyms of bullet include dumdum, dumdum bullet, full metal jacket, rifle ball.

bullet (noun)	rubber bullet (noun)
(baseball) a pitch thrown with maximum velocity	a bullet made of hard rubber; designed for use in crowd control
a projectile that is fired from a gun
a high-speed passenger train