CMMG Mk4/AR15 Upper Receiver Assembly – Armor Black

Description

CMMG has been putting out solid AR parts for years, and this Mk4/AR15 upper receiver in Armor Black is a great example of their no-nonsense approach. It ships as a stripped upper machined from 6061-T6 aluminum and finished in a tough Cerakote coating that holds up to handling and the occasional bump at the range. The look is deliberately minimal since it skips the forward assist, which trims a little weight and gives the receiver a clean, uncluttered profile that pairs nicely with modern handguards and optics.

What Sets This Mk4 Upper Apart

The standout detail here is the enlarged ejection port. That extra clearance means this particular upper plays well with pistol-caliber builds, so it can handle 9mm, 10mm, .40 S&W, and .45 ACP setups. It's also compatible with CMMG's MkG, MkGs, Mk10, Mk9, and Mk17 lower receivers, which makes it a flexible starting point whether you're going traditional 5.56 or building a PCC. One thing worth knowing up front: it is not compatible with CMMG's fixed ejector Radial Delayed Blowback bolt carrier groups.

Building on the CMMG Mk4/AR15 Receiver

Up top you get a full-length mil-spec 1913 Picatinny rail for mounting optics, red dots, or a carry handle, plus M4 feed ramps machined in for smoother, more reliable feeding. It arrives with the ZEROED polymer dust cover and brass deflector already installed, so you're just adding your barrel, handguard, and bolt carrier group to get rolling. Since it drops onto any mil-spec AR15 lower, it slots easily into an existing parts collection.

Why the Armor Black Finish Works

Armor Black is the classic choice for a reason. It matches just about any furniture and hardware you throw at it and keeps a build looking cohesive without any fuss. Backed by CMMG's lifetime quality guarantee against material and workmanship defects, this upper is a dependable foundation for a range gun, a plinker, or a build you plan to grow into over time.