The CMMG AR-15 Resolute 300 Mk4 upper receiver assembly in 6mm ARC is a complete, ready-to-fire upper built for precision shooters who want a modern intermediate cartridge without the complexity. Pair this 16-inch upper with any standard AR-15 pattern lower receiver and you've got a rifle that handles inherent accuracy and ballistic efficiency in a compact, practical package. The Resolute platform is CMMG's answer to shooters seeking a flatter trajectory and less recoil than traditional rifle rounds—ideal for target work, hunting medium game, and training where every round counts.

Built on a billet 6061 T6 aluminum upper and finished in Cerakote, this upper ships with everything you need to get running: a carrier group, ambi charging handle, polymer dust cover, and a purpose-built muzzle brake all factory-zeroed and ready. The 16.1-inch 416 stainless steel barrel is crowned with a 1:7.5 twist to stabilize modern 6mm ARC loads, and the rifle-length gas port feeds a non-adjustable 0.750-inch gas block. The M-LOK handguard gives you clean, modular rail real estate for optics, lights, or vertical grips without the clutter of picatinny fore-and-aft.

The 6mm ARC isn't a marketing gimmick—it's an efficient, inherently accurate cartridge that's gained serious traction with long-range competitors and hunters who appreciate less wind drift and faster follow-up shots. At 25 inches overall and 4.3 pounds, this upper is lean enough for all-day carry and CQB-adjacent work, yet the 16-inch platform delivers velocity and dwell time you'd expect from a purpose-built rifle. Whether you're building a precision 6mm ARC rifle from scratch or converting an existing lower, the Resolute 300 Mk4 is a complete, no-compromise upper that respects your time and your ammunition budget.

The CMMG AR-15 Resolute 300 Mk4 upper receiver assembly in 6mm ARC is a complete, ready-to-fire upper built for precision shooters who want a modern intermediate cartridge without the complexity. Pair this 16-inch upper with any standard AR-15 pattern lower receiver and you've got a rifle that handles inherent accuracy and ballistic efficiency in a compact, practical package. The Resolute platform is CMMG's answer to shooters seeking a flatter trajectory and less recoil than traditional rifle rounds—ideal for target work, hunting medium game, and training where every round counts.

Built on a billet 6061 T6 aluminum upper and finished in Cerakote, this upper ships with everything you need to get running: a carrier group, ambi charging handle, polymer dust cover, and a purpose-built muzzle brake all factory-zeroed and ready. The 16.1-inch 416 stainless steel barrel is crowned with a 1:7.5 twist to stabilize modern 6mm ARC loads, and the rifle-length gas port feeds a non-adjustable 0.750-inch gas block. The M-LOK handguard gives you clean, modular rail real estate for optics, lights, or vertical grips without the clutter of picatinny fore-and-aft.

The 6mm ARC isn't a marketing gimmick—it's an efficient, inherently accurate cartridge that's gained serious traction with long-range competitors and hunters who appreciate less wind drift and faster follow-up shots. At 25 inches overall and 4.3 pounds, this upper is lean enough for all-day carry and CQB-adjacent work, yet the 16-inch platform delivers velocity and dwell time you'd expect from a purpose-built rifle. Whether you're building a precision 6mm ARC rifle from scratch or converting an existing lower, the Resolute 300 Mk4 is a complete, no-compromise upper that respects your time and your ammunition budget.