fastball vs bullet

fastball and bullet both are nouns.

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

As nouns, fastball and bullet are synonyms defined as:

Other synonyms of fastball include heater, hummer, smoke.

fastball (noun)	bullet (noun)
(baseball) a pitch thrown with maximum velocity	(baseball) a pitch thrown with maximum velocity
	a projectile that is fired from a gun
	a high-speed passenger train