Surefire has new 60 and 100 (I think) round mags for the AR platform. They're even in the various military "catalogs" that float around commands. You make a good point about the piston/auto fire thing though.
I may very well be mistaken, but I believe the standard issue ammo "over there" is 62 gr M855 penetrator ammo. Still 5.56, and I'll let people with more knowledge than me (which is minimal) comment on that.
My last deployment we got no 62 green tip stuff at all. Standard ball ammo, I had some loads I made up before hand that I worked up for a NATO chamber, did them with 69 hornadys and it was the tits shooting out past 300m. Also had to field my own ACOG out there as we only had 4 in theatre issued. Kind of a joke really. The upside of being a combat engineer was having plenty of bang to haul around. Either way our standard load out was having a PAIR of M240B for every squad. Any terrain we were in there was one with each of my fire teams.
OIF 2004 we got the 62 grain stuff though. It didn't make much difference there as I was with TF Baghdad and anytime a fire fight broke out the trucks were nearby with M240s. They'd rock out while we got cover. Sadly, my brand new model out of the box took a round right through the gas chamber while I was returning fire and I got a clapped out model. As well as my M4 taking a beating and I swapped out for the A2 by FN. Which I must also say was much more accurate with a full length barrell. Or at least I seemed to stay on target better because the front sight blade was much more narrow looking down the A2, could have been mental.
I can't say enough about having a larger round available. For instance I had to stop a car after an IED strike that was about 600m out, piece of cake with the M240. Saw a guy try to do the same with a M249 and he accidentally shot the guy standing about 10 m to the side.
When I was up in the valley in OEF the SDMs were getting a 7.62 AR variant issued out. Later on down in RC south saw quite a few marines toting M14s as well. 04 OIF army was giving one to each platoon as well but I'm sure we couldn't wield them nearly as well as those Marines. The few I saw shoot were some dead eyes. My point is there is definitely a NEED for a large caliber weapon for both suppression and accuracy.
FWIW from reading about the M27 I think it will be a great rifle and it seems to fit the bill replacing the M249 nicely.
Preparing to be corrected by Statesman now!! Just kidding man...but I know you'll have some good input on the caliber issue.