The GL-MAG buttstock by FAB Defense incorporates an underside magazine carrier well that accepts standard AR mags. The magazine is centered in the stock and doesn’t interfere with positioning the gun on the shoulder. An ambidextrous quick-release button releases the mag for a fast change and then accepts the empty. This eliminates the need for a separate magazine holder strapped on the side of the stock and keeps a spare at the ready.
With a 10-round magazine in the well, as shown in the photo, the stock is nearly the same size and configuration as a standard carbine stock. Obviously, larger magazines will protrude further from the underside of the stock. Larger mags could act as a makeshift monopod, I guess. I personally would recommend against the use of drum magazines with this stock.
Besides the mag carriers, the stock itself is otherwise fairly standard. It has a QD quick detach swivel for sling attachment and has a patented “anti-rattle” mechanism that allows it to fit both mil-spec and commercial buffer tubes. It’s available in black, olive drab green, and desert tan.