Two things all sniper rifles have in common: long range and high accuracy. Snipers usually are long rifles with scope attached to them even in normal status. Their long range makes them perfect for defending objectives, and killing off close range combat classes such as Juggernaut with ease. Some of them are also capable of one-shot-kills.
But they also have weaknesses, such as fire rate. Most of them are semi-auto or even require locking before another shot. If you have problems with this, attach Burst Module. Exception is G36R. It is actually an assault rifle in real combat, and therefore it is full-auto with fire rate of FAL OSW with auto trigger, but also slightly decreased accuracy.
Snipers are the best choices for Sniper class, as they are very vulnerable to close combat due to their low health. Rail Gun is the only better choice. Gunslingers are powerful with Snipers too, especially Calamity. Rate of fire boost killstreak increases Sniper's low RoF (But Gunslingers have terrible accuracy and therefore not suitable for sniping. Also, Gunslingers cannot use Rail Gun). Combined with Burst Module, you can score kills with one shot (although Burst Module isn't always the best, as there is too much ammo expended per click). Always aim for the head for headshots and quick kills.