I use the Surefire M600 scout on mine. A bit more of a narrow beam with longer throw. I used to use the M300, but I need the throw when trying to put through ambient light such as a parking lot, and still getting inside of a car so as to ID a threat.
Agency owned rifles have been switched over to Surefire X300U B's. They are much improved over what used to be on the gun, But the switch is difficult to use compared to the M600. And I don't like the throw for getting inside of target vehicles from a position of cover, compared to my M600.