My boy solomon slipped me this link that shows the picture above of a Marine infantryman using Magpul PMAGs (30-round) for his rifle. He's also got a full loadout of them in his chest rig.
I'm going to do some official inquiries, but as far as I know the PMAG isn't standard issue gear for Marine infantry units. This looks to me like a unit purchase -- and a smart one at that. I've also seen Soldiers equipped with PMAGs in Afghanistan (see below), despite the fact that PEO Soldier recently announced the fielding of a .
Also be sure to take a look at the video I shot of the EMA Tactical countdown magazine, another spin on the "know your load" mantra.