CMMG's Barrel Sub-Assembly is ideal for building your custom AR-15 upper group. It comes with properly installed M4 barrel extension. Accuracy matters with any build, and with this 16.1 Barrel chambered in .350 Legend there's no need to worry. Made from 4140CM steel with a nitride finish for superior durability. Works with CMMG's Mk4 (AR15) Uppers.