The 5 5.7x28mm Barrel and BCG Kit allows you to build an AR in 5.7x28mm using CMMGs patented Radial Delayed Blowback operating system. Unlike typical direct blowback systems, Radial Delayed Blowback not only reduces the weight in your BCG but it results in less felt recoil than a straight blowback system. CMMGs 5.7x28mm Barrel and Bolt Kits are designed to be assembled into a mil-spec AR15 Upper and work with either a CMMG Mk57 lower receiver or with a standard Mk4/AR-15 lower receiver (when using CMMG 5.7 AR Conversion Magazines).