Guns CMMG now offering 5.56mm caliber option for AR15 pistols Recoil Staff July 11, 2014 0 COMMENT CMMG is adding a 5.56mm option to its line of AR pistols. In addition to the new caliber, the Mk4 K AR Pistol features a 12.5 in. barrel (vs. the 8.2 in. versions in .300 AAC Blackout and 9mm). The new Mk4 K comes with a Magpul MOE grip and stock, an A2 muzzle comp and an ambi rear sling mount. The Mk4′s rails are RKM11 free-floating KeyMod (slots at 3, 6 and 9:00 positions) with a Pic rail across the top. As with other CMMG firearms, the Mk4 K AR Pistol comes with a lifetime guarantee against defect in material or workmanship. Learn more on their website, Facebook page, YouTube channel or via Twitter.