You may find the answers to your questions here: http://www.reddit.com/r/AskHistorians/comments/185ibd/how_long_has_the_concept_of_the/

I think the problem with a pre-gunpowder sniper is that it's sort of hard to "snipe" a target past 50-100 yards if they have time to see the projectile and step out of the way or raise a shield. You might have luck with a high velocity crossbow but that still doesn't really come close to a gun