Quickly becoming a fan favorite over the last several years, 5.7x28mm has found favor with those looking for a fast, lightweight round that offers excellent penetration at short ranges with minimal recoil. The CMMG Mk57 platform is designed exclusively to accept FN Five-seveN style magazines without sacrificing essential AR functionality, like last round bolt hold open. Not only can you have the same round in your pistol and carbine, but you can also use the same magazines for both!