The sniper rifles are very effecitve against armoured enemies, they will knock the enemy down for a few seconds, then the enemy will get back up and continue to attack. The best place to aim with a sniper rifle at a heavy armoured enemy is their back where there is almost no armour.
Sniper Rifles are included in the special weapons section, also included is an extensive list of all upgrades:
The M21 Sniper Weapon System is portrayed as the M14 in Army of Two. It is the standard issue sniper rifle for all SSC troops. At its Base, the M14 has medium damage, high accuracy, and medium Aggro.
The H&K MSG-90 is portrayed in game under the same name. At its Base, the MSG-90 has medium damage, high accuracy, and medium Aggro.
The Walther GmbH DSR-1 is portrayed in the game under the same name. At its Base, the DSR-1 has medium damage, medium accuracy, and low Aggro.
The Dragunov sniper rifle is portrayed in-game as the SVD.
The M107 variant of the M82 is used in Army of Two, and is referred to as the M107 Light 50. At its Base, the M107 has extreme damage, extreme accuracy, and high Aggro. While it can be used as a sniper rifle, it can also be used to take out helicopters.