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.

UN Company has sent in this video of the Andax Works Lancer L5 magazine made specifically for the Systema PTW M4s or M16s at affordable rates... "Dear All, UN Company is going to show you the polymer magazine for Systema PTW M16/M4 Rifles. This Andax Works Lancer L5 magazine is translucent with fully trademarks and logos which is licensed by Lancer Systems. It can store 120 rounds bbs and it comes with 5 magazines in a box set.