They developed a magazine to use for Systema PTWs, and now they have magazine shells for WE Gas Blowback rifles. You'll need to use the original magazines and insert them into these shells. Available at Airsoft Extreme for US$19.99... "Andax Works Officially Licensed TangoDown ARC Magazine Shell for the WE M4 and SCAR GBB Rifles are in stock NOW.  These are polymer magazine body shells ONLY.

The patent pending ARC magazine was developed by TangoDown in cooperation with Navy Special Warfare Command and USSOCOM as part of the SCAR program to improve “drop-free” performance, feeding reliability, durability, and weather resistance over existing M16 and SCAR rifle magazine solutions. Click on the image to get more info and to order yours today!"