bullet vs rifle ball

bullet and rifle ball both are nouns.

Word	Noun	Adjective	Verb	Adverb
bullet	Yes	No	No	No
rifle ball	Yes	No	No	No

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

bullet: a projectile that is fired from a gun
rifle ball: a bullet designed to be fired from a rifle; no longer made spherical in shape

Other hyponyms of bullet include dumdum, dumdum bullet, full metal jacket, rubber bullet.

bullet (noun)	rifle ball (noun)
(baseball) a pitch thrown with maximum velocity	a bullet designed to be fired from a rifle; no longer made spherical in shape
a projectile that is fired from a gun
a high-speed passenger train