The S&W M&P15-22 Gas Block, while non-functioning, is a must-have if you want to install a standard 2-piece handguard on your Smith & Wesson M&P15-22 rifle. This gas block also features a single Picatinny rail (MIL-STD-1913) at the shorter "Gas Block" height. NOT "Rail Height" One advantage of gas block sights is that they move with the barrel as it flexes from heat, pressure,