Yes, the first thing that many gas blowback rifle users will react is the 39-round magazine capacity of the PMAG developed by G&P under licence from Magpul PTS. That's the way how gas blowback magazines for M4 rifles are and until a new system is developed, then we have to live with such limitations. The only option you need is to have fire discipline and don't fire until there's a high probability of making it a hit on the OPFOR, which means very much close.

Anyway, if you're after the looks, especially after customising your WA-compatible M4 GBB-R with Magpul PTS accessories, then you can have this at Evike.com with sand or black colour to choose from. That is, if you won't balk at the US$75.00 price. This can fit the WA M4 series, and the G&P WOC M4 series. Check if this can fit with Inokatsu, Jing Gong, and AGM models as they're also based on the WA Gas Blowback system.