This 5.56 chambered 12.5″ Government Profile Modern Series Barrel is machined from 4150 Chrome Moly Vanadium steel with a QPQ Corrosion Resistant Finish and QPQ coated M4 feed ramp extension.Specs- 750″ Gas block journal, gas system length- carbine and a Government profile. 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.