The Firearms Blog has a post on a patent filing from Magpul Industries for a so-called "quad-stack magazine."
Basically, as TFB puts it, the slightly wider magazine accomodates what looks like about 50 rounds using two springs and a divider to weave the 5.56 rounds in.
The design is quite straight forward. A central partition separates two dual staggered round stacks. Two springs are used, a lower stronger spring and a weaker top spring, which are joined by spring slicer.
The magazine has a constant curve geometry. Interestingly, the transition area is asymmetric in order to stagger rounds correctly.
Check out the post for more schematic detail.
I raise this because it adds to the Marine Corps Infantry Automatic Weapon debate that's swirling quietly in the halls of Quantico and the Pentagon. I've heard of grumblings from within the Corps that the IAR is a waste of time and money and doesn't really add to the capability of the fireteam. Primarly complaints stem from the lack of firepower offered in a 30 round magazine. Well, as TFB points out, the Magpul magazine, if it works, could be the answer that corrects the problems with C-Mags and answers the concerns of grunts who want the amount of led the SAW carries.
One thing is for sure, the Corps is still going ahead with the IAR program, which they call the M27 -- it's just a matter of how many and for whom at this point.