shooter vs marksman

shooter and marksman both are nouns.

Word	Noun	Adjective	Verb	Adverb
shooter	Yes	No	No	No
marksman	Yes	No	No	No

As nouns, marksman is a hyponym of shooter; that is, marksman is a word with a more specific, narrower meaning than shooter:

shooter: a person who shoots (usually with respect to their ability to shoot)
marksman: someone skilled in shooting

Other hyponyms of shooter include crack shot, sharpshooter, gun, gunman, trapshooter.

shooter (noun)	marksman (noun)
a large marble used for shooting in the game of marbles	someone skilled in shooting
a person who shoots (usually with respect to their ability to shoot)
a professional killer who uses a gun
(sports) a player who drives or kicks a ball at the goal (or a basketball player who shoots at the basket)
a gambler who throws dice in the game of craps