In Essex county, VA, pre-war (and in reaction to the terrorist John Brown raid), a volunteer militia company was raised, and called itself the "Essex Sharp Shooters." In 1861 it became a component of what became the 55th Virginia Infantry Regiment as Co. F (Capt. Thomas M. Burke). I think it significant that the unit insisted upon Sharp Shooter (two words), rather than "sharpshooter," and used the letters "E S S" as a distinctive marking on its pre-war headgear. This is usually ignored in later writing, but it suggests to me an earlier form, modified over time -- and, of course, pre-dating "sniper."