PMAGs are no longer approved for use by the USMC, only the EMAG now.
Something to do with the IAR's magwell? ....
Yep. The magwell is too tight for PMAGs.
The PMag is still being fielded for M4s. Some USMC units are buying EMags instead due to the introduction of the IAR.
The issue comes from the HKs 416 Magazine well (the IAR is based on the 416) which is longer than any other AR15/M16 magwell.
Why? Well here is the reason behind it.
In 2000, Heckler & Koch, at that time owned by the British small arms manufacturer Royal Ordnance, was contracted to upgrade the SA80 family of weapons. Two hundred thousand SA80s were re-manufactured at a cost of £400 each, producing the A2 variant. This included the famous HK SA80 Steel magazines.
In addition to the regular steel magazine they produced a blank firing version with a sliding round inhibitor.
So this magazine was fielded with the UK military as part of their blank firing kit for ages. The lower edge of the SA80 mag well moves the round inhibitor down so the blank rounds can be fed into the chamber.
Fast forward to 2005 and the HK 416 was being developed they used the same SA80 steel magazines but to make the blank magazines work they had to lengthen the front of the magwell to engage the switch.