PSA AR15 keymod lightweight barrel nut w/ set screws. For use with PSA rails that have a steel alignment insert just below the top rail, and three clamping screws at the bottom base of the rail.
The bottom clamping screws only hold the rail snug to the barrel nut. One allen screw engages a slot in the barrel nut and keeps the rail from linear movement. The smaller allen screw is used to remove pressure from the bottom pinch clamp.
Always remove the smaller allen screw before reassembly, as it can keep the three pinch screws from clamping.