The Collaborative International Dictionary of English v.0.48:

Marksman \Marks"man\, n.; pl. Marksmen. [Earlier markman; mark
   + man.]
   1. One skillful to hit a mark with a missile; one who shoots
      well. especially with a pistol or rifle.
      [1913 Webster +PJC]

   2. (Law) One who makes his mark, instead of writing his name,
      in signing documents. --Burrill.
      [1913 Webster]

WordNet (r) 3.0 (2006):

marksman
    n 1: someone skilled in shooting [syn: marksman,
         sharpshooter, crack shot]

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "marksman":
   Admirable Crichton, Nimrod, Zouave, adept, archer, artilleryman,
   artisan, artist, attache, authority, bersagliere, bowman,
   cannoneer, carabineer, chasseur, connaisseur, connoisseur,
   consultant, cordon bleu, crack shot, craftsman, dead shot, deadeye,
   diplomat, diplomatist, dogface, doughfoot, elder statesman,
   experienced hand, expert, expert consultant, expert rifleman,
   foot soldier, footslogger, fusileer, good shot, graduate,
   grenadier, grunt, gun, gunman, gunner, handy man, hunter,
   infantryman, journeyman, light infantryman, markswoman, musketeer,
   no slouch, paddlefoot, politician, pro, professional, professor,
   proficient, rifle, rifleman, savant, shark, sharp, sharpshooter,
   shooter, shot, sniper, statesman, targetshooter, technical adviser,
   technician, toxophilite, trapshooter