rifle ball vs bullet

rifle ball and bullet both are nouns.

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

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

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

Other hypernyms of rifle ball include slug.

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