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. Redesigned with a brand-new look, CMMG's 5.7 AR Conversion Magazines feature a number of improvements. From the stylized texture grip and "5.7x28mm" engraving on the sides to the upgraded feed lips, these magazines are built to exceed your expectations. 5.7 AR Conversion Mags are available in 10 or 32 round capacity and are designed to work with CMMG 5.7x28 Radial Delayed Blowback Uppers and Mk4/Mil-Spec AR15 lower receivers.