The Accuracy Systems adjustable gas block allows you to tune your gas system for optimum performance. By adjusting the gas system you can; adjust ejection distance of the brass and increase recovery, reduce wear on your operating rod and increase accuracy by tuning the harmonics of your rifle. The gas block can also be turned off completely so the rifle will operate as a single shot. Easy installation, replaces the factory gas block with four screws. Fits 0.562" diameter barrels on Pre-580 series rifles.