CMMG’s 5.7 AR Conversion Magazine holds 5.7x28mm rounds and fits in any standard AR15 lower receiver. To use, simply attach a CMMG 5.7x28mm Radial Delayed Blowback complete upper receiver to an existing AR15 lower. Then, insert CMMG’s 5.7 AR Conversion Magazine to safely feed 5.7x28mm ammunition. CMMG recommends using a standard carbine buffer and action spring to reliably cycle the action in this configuration. The 5.7 AR Conversion Magazine also retains last round bolt hold-open functionality, allowing for faster reloads.