Just the opposite of my experience with ARs. I don't know off hand how many mags I have for my ARs, but it's got to be 50>. Everything from the $30> mags to surplus Asian for a couple bucks. It's been my experience, that if they have good springs and followers, and the mag tube is undamaged, they all work great. Not sure what AR you have, but most mags that come with a new gun are made by an aftermarket manufacturer.
Savage