The long barreled, new Mk4 K 5.56 AR pistol from CMMG


For enthusiasts of CMMG products, the new arrival – Mk4 K 5.56 AR pistol is sure to serve as a fantastic addition to their armory. This latest addition to their AR-15 pattern pistol line-up comes with quite a number of interesting and valuable features. As the name indicates, it has been chambered for 5.56 NATO and is equipped with a much longer barrel than their other 9mm pistols and the 300 AAC Blackout.

Impact of longer barrels

The barrel of Mk4 K is about 12.5 inches, a significant increase from 8.2. This certainly enhances the muzzle velocity. Barrels that are shorter or cut down too much generally decrease the pace of cartridges like 5.56 NATO and .223 Remington. The increase in length of the barrel will most certainly augment the motion and flight path of the bullets placing it a notch above other guns with shorter barrels.

Most ARs that come with shorter barrels have issues with pressurizing. Due to slow-burning and low-pressure ammo, the reliability factor is considerably less in ARs with short barrels. Mk4 K 5.56’s longer barrel that has a medium weight contour and a 1-in-7 twist, thus assumes great importance in enhancing the consistency of the pistol.

Features and Characteristics

  • Grip and Handguard: the Mk4 is equipped with a Magpul MOE pistol grip and stock and a remarkably lengthy and free-floating handguard. This uniquely long handguard comes with three, six and nine o’clock KeyMod slot positions which support mounting of AR-15 accessories directly onto the handguard.
  • Rails: The Mk4 K 5.56 AR is equipped with CMMG’s very own RKM11 rail. For increased compatibility and user-friendliness, the company provides users with separate 5-slot adapter rails for Picatinny accessories. The 12 o’clock Picatinny rail can be used for appending standard iron sights or other similar scopes.
  • Optics and Sling Mount: CMMG offers great flexibility for users in the optics department as they are allowed to add their own preferred sights. This also significantly reduces the original cost of the pistol. It is also equipped with an ambidextrous rear sling mount end plate and A2- type flash.
  • Weight and length: The weight of the pistol is a remarkable 5.3 pounds when unloaded and has a length of about 28 inches.
  • Build and Ammo: The receivers made of forged 7075-T6 hard anodized aluminum while the specifications of the fire control group and other components are standard MIL-SPEC. The Mk4 comes with a 30-round Gen 2 Magpul PMAG standard but does not come with a stabilizing arm brace even though it could certainly do with one. May be CMMG will rethink its strategy with respect with shipping arm braces in future.
  • Price Tag: Even though we would all wish for a much economical under-$1000 version, the present price tag of Mk4 K 5.56 AR is about $1049. As with all other CMMG products, this also comes with a lifetime guarantee.

For more information on the pistol and CMMG’s other products or to purchase them visit


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