CM, I am going to order one of these. It allows you to use SW 15-22 mags in that CMMG .22 upper. Plus, it has a working last round bolt open device.
Better-Mag S&W .22 Mag Adapter : BETTER-MAG S&W .22 RIMFIRE MAGAZINE ADAPTER | Brownells
Better-Mag S&W .22 Mag Adapter : BETTER-MAG S&W .22 RIMFIRE MAGAZINE ADAPTER | Brownells