That depends on how far away you are, almost every sniper rifle kills within its sweet spot range.
If you´re performing a headshot, every sniper rifle kills with 1 shot, no matter which distance.
Here´s a video by Jackfrags, it explains the graph that you can see when you look at your rifles:
Just pick up the Tank Hunter elite class and one shot everything :D 😉
Community Admin+1 for Tank Hunter.
It's by far my favorite Elite Class, and with that absurd rifle you don't need to deal with petty concerns like headshots or sweet spots.
As for sane weaponry, as folks are saying check your sweet spot graph and pick the appropriate one for the map/engagement range.
You might also want to think more about your choice of sidearm when playing as Scout. Having a pistol you're comfortable with can help you with finishing off wounded targets, or with getting away when you're ambushed. A quick rifle chest shot then panicky pistol spam has saved me more than once.
