Description
The CMMG Mk4/AR-15 Upper Receiver Assembly is a drop-in platform upper for any mil-spec AR-15 lower. It is machined from forged 7075-T6 aluminum, fitted with M4 feed ramps, and finished in a durable Midnight Bronze Cerakote. The assembly ships complete with the forward assist and CMMG ZEROED polymer dust cover already installed, so builders can drop it on a lower without any small-parts bench work.
The defining feature of the Mk4 platform is its enlarged ejection port, which lets a single upper run a wide range of pistol-caliber conversions – including 9mm, 10mm, .40 S&W, and .45 ACP – in addition to standard rifle calibers like 5.56x45mm NATO. That versatility makes this upper equally at home as the foundation for a custom build or as a direct replacement for a worn factory upper.
Pair it with a CMMG barrel, handguard, and bolt carrier group in the same Midnight Bronze finish for a coordinated custom build, or use it as a premium replacement part on an existing AR-15.
