This is much more difficult then it sounds, it can be easily acquired in Team Big Ass Chains while boosting for other achievements. This is a good one to work on while doing Kingpin or Chain Gang. Depending on the number of players in the game it won't take long. In a 2v2 session we were averaging 30 kills per match.
Team Big Ass Chains has 3 maps. Below are the locations of the sniper rifle.
1) Garage- top floor of parking garage, it is on the left side if you are looking down the ramp.
2) Arena- On the center metal structure look for the "foreign powers" ad, turn around and head under the open metal girder bridge. The sniper rifle will be inside a concrete tube on the left.
3) Streets- on the street level this will be in the south dead-end street behind a light post.