#Mk57

Here’s what CMMG had to say:

CMMG continues on the 5.7×28 revival path.  Last year, we reviewed CMMG’s FiveSeven magazine compatible 5.7×28 Banshee.  Further reinforcing the 5.7×28 revival is CMMG’s new Mk57 Upper Conversion kit for standard AR lowers.  Of most import is the fact that CMMG developed a 40 round 5.7×28 magazine to ship with the Mk57 that fits into one’s AR15 lower without any modification.

CMMG’s Mk57 series is their line of 5.7x28mm firearms.  Their “Banshee”  model incorporates their shortest barrel in the caliber (5″), as well as a Tailhook PSB.  All Mk57 models use CMMG’s Patent Pending Radial Delayed Blowback (RDB) system found in their GUARD models.  The RDB blowback bolts have a head that is forced to fully rotate to unlock, causing the bolt carrier group to slow down during the firing cycle.  This allows CMMG to use lighter bolts, and lighter, slower bolts mean less recoil.   CMMG’s video illustrates the technology quite nicely.