I only have HiCaps and stock HiCap wiggles around a bit but works, however the DBoys Metal HiCaps that I got with my GR16 works like a charm. I also find that their black matches more with the gun instead of the black-ish green that the G&G Stock magazine is and is also shorter so it doesn't look as awkward as a long magazine sticking out. However, I'm not sure what LoCaps or RealCaps would work with a GR16 and fit snugly, but I can tell you that the DBoys HiCaps @ VA fit nice and snug.
If you can't kill then you are always subject to those who can
Revenge is sweet, but it is not nourishing.