rubber bullet vs bullet

rubber bullet and bullet both are nouns.

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

As nouns, bullet is a hypernym of rubber bullet; that is, bullet is a word with a broader meaning than rubber bullet:

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

Other hypernyms of rubber bullet include slug.

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