I just got a 5-pack of King Arms metal M4/M16 mags last week. I've tried them all to ensure functionality, and they seem to work fine without any feed issues.
I also have a box of King Arms plastic AK47 mags. I don't have any feed issues with those either, although I had to dremel down the lip that engages into the mag catch, since they were a bit too thick and the mag catch would't lock onto them. Now that I've ground down that lip, no problems.
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