It has been reported on the ARFCOM and Lightfighter forums that some USMC battalions are banning or restricting the use of Magpul PMAG magazines because they are incompatible with the M27 IAR. The PMAG does not work with the HK416 (it does not drop free and needs to be pulled out), and its derivative the M27 IAR, because of slightly different lower magwell geometry.

The Magpul EMAG (Export Magazine) was designed by Magpul to work with non-AR-15/M16/M4 rifles that use the STANAG magazine, such as the SA80 and G36, and should work fine with the M27 IAR.

[ Many thanks to jdun1911 for emailing me the link. ]