AC-ACW Accuwedge will instantly tighten loose upper & lower receivers due to tolerance. Will fit any AR15, M16 and M4 rifle, carbine or pistol. Fits near the rear takedown pin and keeps the upper receiver section snug and tight against the lower receiver. This product helps promote better accuracy by making the upper and lower receiver fit tighter and will also decreases vibration and prevent wear. Easy install, no gunsmith required. Just open your upper receiver and drop this rubber wedge into the rear of the lower receiver behind and underneath the rear takedown pin.