machine gunner vs cannoneer

machine gunner and cannoneer both are nouns.

Word	Noun	Adjective	Verb	Adverb
machine gunner	Yes	No	No	No
cannoneer	Yes	No	No	No

As nouns, machine gunner and cannoneer are synonyms defined as:

Other synonyms of machine gunner include artilleryman, gunner.

machine gunner (noun)	cannoneer (noun)
a serviceman in the artillery	a serviceman in the artillery