The factory mag that came with mine functioned better out of the box than the two after markets that I bought with it. That being said, it is a poly mag and I'm not sure how durable it will be vs. steel over the long haul. There are several fixes for the after market that will make it function at 100%, though. I also have to agree with PepeLapiu; if you rack it with authority you'll avoid a lot of problems.
Check out this thread