"If the Attacker benefits from Good Cover, or his cover obscures his view, apply a –2 dice pool modifier to any attacks."
This is perhaps one of the most frustrating rules about sniper rifles. If you have good cover, even if it doesn't obscure your view in any way, for example, you're firing through an open window, nice and comfortable, you get a -2 to hit.
Now I understand why they needed someone on the grassy knoll. Oswald had a -2 to hit Kennedy.