The Ballistics Advantage 5.56 M4 Carbine Length 14.5" Modern Series Barrel is machined from 4150 Chrome Moly Vanadium steel with a QPQ Corrosion Resistant Finish and QPQ coated M4 feed ramp extension. *All NFA/ATF Rules Apply. You may use this barrel with a lower receiver that is a registered SBR, Machine Gun, or Pistol. Possession of this barrel along with a Non-NFA lower receiver is Prohibited, except when installed on a Pistol configuration firearm.