The TAR-21 with the holographic sight, with a SPAS-12 as a secondary. Slight of Hand Pro, Stopping power Pro, the scrambler thingy. Use this setup with the SCAR-H as well, but with Scavenger pro instead of slight of hand pro.
I feel that sniping is kinda useless in Modern Warfare 2, it's not really what it was in Modern Warfare.