CMMG has added a new caliber to their line of AR pistols. The MK57 is an AR pistol chambered in 5.7x28mm. It is fed using the magazine from the FN Five-seveN pistol magazine.
The Five-seveN magazines come in 10 or 20 round capacities. You can get aftermarket 30 round magazines. The 5.7x28mm cartridge was developed by FN Herstal for their P90 PDW weapon. It is a small compact round that looks like a miniature 5.56x45mm round. Before the CMMG MK57, there was an option to have an AR chambered in 5.7x28mm. The AR57 is a proprietary upper that uses P90 magazines. I wrote a review on the AR57 here.
One benefit to the AR57 is the increased capacity. P90 magazines can hold up to 50 rounds. The downside is a completely new operation. The magazine release, charging handle and even the ejection is different from your standard AR platform. Due to the long magazine, there is a limit to how small an AR57 can be. This is what sets the CMMG MK57 apart from the AR57. It is in a sense a pistol caliber carbine. Since it uses Five-seveN pistol magazines, you can carry the pistol as your sidearm and use the same magazines across both weapons. Another distinct difference between the MK57 and the AR57 is the action. The AR57 is direct blowback while CMMG is capitalizing on their radial delayed blowback for softer recoil.
CMMG offers three different versions of the MK57. There are two 8″ barreled pistols and the Banshee which has a 5″ barrel. The MK57 starts at a price of $1,349.95 without a stabilizing brace. Or you can spend $1,399.95 for an MK57 with a Shockwave Blade. The Banshee is the most expensive out of the three at an MSRP of $1,549.95 and it comes with a Mod 2 Tailhook.
You can check out more about the MK57 on CMMG’s website.