The term sniper was first attested in 1824 in the sense of the word "sharpshooter". The verb "to snipe" originated in the 1770s among soldiers in British India where a hunter skilled enough to kill the elusive snipe was dubbed a "sniper"
During the American Civil War, the common term used in the United States was "skirmisher". Throughout history armies have used skirmishers to break up enemy formations and to thwart the enemy from flanking the main body of their attack force. They were deployed individually on the extremes of the moving army primarily to scout for the possibility of an enemy ambush. Consequently, a "skirmish" denotes a clash of small scope between these forces. In general, a skirmish was a limited combat, involving troops other than those of the main body. The term "sniper" was not in widespread use in the United States until after the American Civil War.
The term "sniper" has been used in more serious tones especially by media in association with police precision riflemen, those responsible for assassination, any shooting from all but the shortest range in war, and any criminal equipped with a rifle in a civil context. This has rather expanded the meaning of the term. It has also given the term "sniper" mixed connotations. Official sources often use more positive connotative terms to describe snipers, especially for police snipers: "counter-sniper", "precision marksman", "tactical marksman", "sharpshooter", "precision riflemen", and "precision shooter". Some of these alternatives have been in common use for a long time; others are closer to undisguised euphemisms.
More Info From Wikipedia
Barrett .50 Cal (Light .50)