round shot vs cannonball

round shot and cannonball both are nouns.

Word	Noun	Adjective	Verb	Adverb
round shot	Yes	No	No	No
cannonball	Yes	No	No	No

As nouns, round shot and cannonball are synonyms defined as:

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

Other synonyms of round shot include cannon ball.

round shot (noun)	cannonball (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