Nylon tipped tension set screw - Aero Precision Nylon Tipped Tension Set Screw

This is a great addition to your lower. My understanding is that it will improve accuracy by securing and aligning the upper to the lower and prevent scope misalignment. Make sure your lower is fitted with the threaded channel below the rear takedown pin. If you don't see the channel, then your lower wasn't made with this feature and you can't screw this set screw up through the channel above the grip (grip must be removed to install). Consider an Accu-wedge if you don't have this feature manufactured with the lower.