It would be best if you did something to vastly differentiate the two, rather than have just small differences.
Some suggestions:
Make it use a VERY large caliber round compared to the Barrett .50 (.50 BMG), like maybe the 20mm Vulcan. With armor piercing and explosive ammunition variants.

For balance purposes, such a rifle would be very heavy, have a slow rate of fire & target acquisition, and have limited ammo, like 3-5 rounds tops. Also, have an integrated bipod that cannot be uninstalled. A beast of a rifle like this would HAVE to be fired prone or balanced on a surface.