This is not about them being a one shot, because Sniper Rifles should be high damage and SKILL REWARDING. That's the key, and right now they don't require skill.
First of all, make the time to scope in slightly longer, it's currently way to easy to just one shot someone who easily has the jump on you, killing people that flank you should require good movement skills, and possibly switching to secondary.
Secondly, make them less accurate while moving. If you have a sniper rifle you can sprint around the map, and quickly sprint around and kill people even faster than with an SMG. Snipers should punish movement and reward keeping to a stationary location.
Thirdly, reward precision aiming. It seems that shooting anyone above the legs is an instant kill, but it should be chest and above, not stomach. This makes it so that you have to aim more carefully.
With these 3 in combination, sniping will be much more of a actual sniping like role requiring more skill. Rewarding precision aim, playing defensively, and less quick scoping.