round shot vs cannon ball

round shot and cannon ball both are nouns.

Word	Noun	Adjective	Verb	Adverb
round shot	Yes	No	No	No
cannon ball	Yes	No	No	No

As nouns, round shot and cannon ball are synonyms defined as:

round shot and cannon ball: a solid projectile that in former times was fired from a cannon

Other synonyms of round shot include cannonball.

round shot (noun)	cannon ball (noun)
a solid projectile that in former times was fired from a cannon	a solid projectile that in former times was fired from a cannon