CMMG Mk4/AR-15 Upper Receiver Assembly Features The CMMG Mk4/AR-15 Upper Receiver Assembly starts with a billet 6061 T6 aluminum receiver machined from a solid block rather than forged for tight tolerances and a clean uniform appearance. Chambered for 5.56 NATO / .223 Rem the upper pairs with a 14.5 steel barrel carrying a 1:7 twist rate suited to stabilizing a wide range of bullet weights commonly used in this caliber. The steel barrel construction offers durability under repeated firing while keeping costs reasonable compared to exotic barrel materials. Builders assembling an AR-15 or Mk4 platform rifle get a drop-in upper that pairs cleanly with a compatible lower receiver. Finished in a standard Cerakote Tungsten coating the upper offers the corrosion resistance and durability Cerakote is known for along with a distinctive gray-toned appearance that stands out from a typical black or anodized finish. That finish is applied to the billet receiver itself giving the whole upper a consistent factory-quality look. Because it is a billet design rather than forged the receiver can hold tighter dimensional tolerances throughout the build. For shooters building a custom Mk4 or AR-15 pattern rifle this upper receiver assembly offers a durable already-finished foundation to build around. CMMG Mk4/AR-15 Upper Receiver Assembly Specifications Manufacturer: CMMG Model: Mk4/AR-15 Upper Receiver Assembly Caliber: 5.56 NATO / .223 Rem Barrel Length: 14.5 Barrel Material: Steel Twist Rate: 1:7 Receiver Material: Billet 6061 T6 Aluminum Finish: Cerakote Tungsten Available At: GrabAGun