CMMG 5.56 16 Inch M10 Upper Receiver has a mid-length hand guard. 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.