This item instantly tightens loose upper & lower receivers. Will fit any AR15, M16 and M4 rifle, carbine or pistol. Fits near the take down 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 prevents wear. Easy “no gunsmith” installation. Just open your upper receiver and drop this rubber wedge into the rear of the lower receiver behind the push pin.