The Magnetospeed M-Series Grip, is a new AR15 grip with modular storage. The grip has a springloaded plate so your stored accessory shoots out with force.
The module is held in by a toe catch. Similar to a heel mag catch but this one is in the front of the grip.
The 3rd magazine will work with 5.56 and .300 blk. It does not have a mag catch notch, it is so short that the mag body sits above the mag catch.
They also make a small LED flashlight that stores inside the grip. There is a plastic clip that attaches to picatinny rails and the flashlight attaches to that plate with magnets.
The grip retails for $32 and below are the modules with prices:
3 round Mag $10.99