For those times you want to build your AR lower group exactly to your spec, the Mk57 Lower Receiver Assembly is here to fit the bill. Designed to accept AR15 accessories like trigger groups, safeties, and grips; this lower receiver pairs with our 5.7x28mm Radial Delayed Blowback upper groups and allows for maximum compatibility between your Mk57 build and other firearms that utilize FN* Five-seveN* pattern magazines.Lifetime Quality Guarantee**Mk57 Lowers are designed for use with CMMG 5.7x28mm Radial Delayed Blowback uppers only**