The Adams Arms Standard Mid Length Piston Kit is a short stroke free floating piston system that is self-cleaning and has a corrosive resistant coating. The kit includes an one-piece bolt carrier, .750" Picatinny gas block with 3 settings, gas plug, upper receiver bushing and installation rod, bolt spring, drive rod with bushing and spring, and a M4 handguard cap. This model also includes an installation DVD and requires a mid gas length 13.241" from barrel extension to shoulder and a .750" gas seat.