A sniper will generally have longer range scopes, slower fire rate then a scout rifle, and deal more damage. It also runs on Special ammo, whereas the SR has white ammo. Its also worth noting I have yet to see an elemental SR, but have seen elemental Snipers ([url=http://i2.wp.com/destinynews.net/wp-content/uploads/2013/03/gozen-c.jpg]example[/url])

a sniper gun has a zoom feature and looks like a traditional sniper gun, the scout looks like a rifle but shoots 1 shot really hard to use when the enemy get harder as you progress