The most effective tactic to use against unarmored, especially used by very good players, is to shoot the target in the torso and let it bleed out. This allows the most kills per magazine, but does not work against targets wearing kevlar because of the decreased damage and the lack of bleeding when a target wears kevlar. The other most effective tactic is to aim for headshots.
The PSG-1 makes a great infiltrator/scout sniper rifle, as it can be silenced. Some scouts use only a PSG and Beretta, to great effect, as if they know the map well, the player can find a very effective sniping position without being noticed too much. The main disadvantages of the psg is its inability to one shot kill against the body. Also many inexperienced players will tend to fire very fast with the psg, expending their ammunition much faster than they need to.