My JG 416 feeds well with all my mags. I use King Arms 68 round mags mostly, but have a couple of mystery brand metal 120 rounders (definitely not King Arms) as well as JG, CA and TM hicaps, and they also all feed without issues.
You may want to PM Sepulcrum about PMags for more info. He bought some for his JG 416 and if I remember correctly, he had to lightly dremel inside the magwell at the front to get them to fit. They have more of a curve than a standard STANAG mag, and because of the slightly different shape of the 416 mag well, they don't seat properly.