The Sniper Rifle is a long-ranged weapon that can be encountered rarely in Area 51. It is wielded by Black Ops forces on occasion, but is usually found in proximity to dead Human Military Forces.
With a well-aimed shot, it is a one-hit kill on both mutants and Black Ops forces, and can be invaluable when in the caverns. The Sniper Rifle does not carry a high amount of ammunition for it's maximum capacity, but can be useful regardless of that, if used well.
It is an incredibly efficient weapon, but can be limited in effectiveness by the close quarters nature of many spaces in Area 51. As such, there are only a few spots in which it is useful.
It can also be used at close range if you need to dispatch an enemy quickly and easily, as it should be fairly easy to get a headshot on a Black Ops soldier, and not too tough to land one on a mutant, either.
Enemies wielding Sniper Rifles are generally easy to locate, and thus do not tend to pose much of a threat, but if one cannot be found quickly it is recommended to find cover and look for the source of the bullets being fired from a safe distance.