missions like the one where you goto a village to talk to the chief and there are snipers on the nearby hills taking pot shots at you, with only open ground between you and them.
Are you talking about the very first mission? Where your squadmates regularly let loose with covering fire to allow you to get from cover point to cover point until you're at the hill side and the snipers can't see you any more?
If so, there's no way you should have been trying to take out the sniper from long range, and the guys even state that they're going to lay down the covering fire for you so you can get closer.
As long as you know where the sniper is (and I didn't even spot him the first time, but remember your binoculars (B)), there's enough cover available.