CMMG 16 Inch 5.56x45 Rifle Length Hand Guard Upper Receiver has an M10 profile barrel. The M10 profile barrel has a 1:7 inch twist ratio. The receiver is part of the M16 Bolt group. Chambered in the NATO 5.56x45mm, save some money with the cheaper Nato round! The receiver is 4150 cold hammer forged. An F marked front sight base. The upper receiver has a low pro gas block that is mid-length.