Its found in the level "Corporate collapse" or something, where you have to mount the turret(tank cannon) to blow up the door, in the beginning of the level you will see one CELL soldier being killed by two cephs, search near the soldier for a scar. This attachment(used in scar only) is same as gauss sniper rifle used by lockhart, takes out most enemies in one shot, also ammo is found in normal crates unlike the gauss rifle.
Thanks. Will get it during my second playthrough with the dx11 stuff....hopefully.