With the issue over the ban of the U.S. Army on Polymer magazines such as the Magpul PMAG already over and soldiers can use these again, it's the U.S. Marines' turn to put a stop to the use of these magazines.  The reason with this ban is compatibility with select weapons, most specifically with the M27 IAR, which is the Heckler & Kock HK416 modified to the specifications of the Marine Corps.

So what's the problem with the Polymer magazine such as the PMAG with rifles such as the HK416? So far the PMAGs work best with the AR platforms such as the M4 and the M16 rifles, thus the U.S. Army took back its policy of banning these for the U.S. Army soldiers. But for other 5.56 rifles such as the SA-80, FN SCAR-L, and most of the H&K 5.56mm rifles, the PMAG doesn't work with these well and feeding issues have been reported.

With this problem, Magpul released the PMAG 30 GEN M3 5.56x45 Magazine that they claim to be compatible with the mentioned rifle. But since this is a new version, it hasn't been tested and gotten the approval of the MARCORSYSCOM and thus the ban remains in place. Some have argued at Soldier Systems (where you can also read the full MARDMIN) that even if the PMAG 30 GEN M3 is compatible with the M27 IAR, visual identification between it and the older PMAGS and EMAGS which are not compatible might pose a problem with the Marines when they need a magazine whilst in combat, especially when operating with other non-USMC units that use polymer magazines too.

This ban might be temporary to allow the USMC to develop guidelines on testing and procurement on Polymer magazines. Until the USMC can ascertain that the Marines won't encounter a failure of the use of Polymer magazines in combat, this ban will be in place.