The only silent sniper in the game is the Supressed M21 which is only available in single player, if you want it on Multiplayer then your gonna have to hack the patch.
I've seen a technique once or twice that works on stationary targets only and i doubt it's very efficient but i've seen it on killcam so here goes;
Scope and hold breathe with sniper change to silenced G3 or Pistol Shoot
like i said i have only ever seen it Once or twice when i have been killed by it.
Never managed it myself.