This complete Upper is only legal if it is put on a Class III firearm or used as a pistol upper. If it is Class III, then this barrel assembly is all you need to convert your 5.56mm AR to 300 AAC BlackOut. The 4150 steel barrel features a black nitride finish (including the bore and chamber), 1:8" twist for super- or sub-sonic use. It comes ready for silencer use, threaded 5/8-24. It includes a thread protector, gas tube, and a steel low-profile press-fit set screw gas block (barrel has set-screw divots for gas block alignment).