I am kind of a fan of the Army's solution. The new block upgrade to the M-4 swaps barrels out for heavier duty ones, gives us back our full-auto, and makes the selector lever ambidexterous. Rather than replacing a SAW with an IAR, they've basically made every carbine into a half-assed IAR and kept the SAW.
I'm not a huge fan of the Beta C-Mags. They have one very important thing going for them, and that is the 100 round capacity. But they jam / misfeed. They require graphite lube every 5 fills. And, as a spoiled shooter, I prefer to wrap my hand around the mag well for CQB, which just doesn't work with a C-Mag. Actually, other than firing from the prone with a bipod, nothing is really comfortable when it comes to shooting with a C-Mag.
Oh, and on my Bushmaster, the C-Mag only locks in when the bolt is to the rear. It locks in solid on my POF, but if I have my finger anywhere near the mag release, it gets squished between the drum and the rifle.
Biggest phony pony is that a C-Mag takes up a ton more vest space than STANAG mags do. I guess the Surefire mags are closer to right for hi-capacity rifle mags. I can't think of a better way that doesn't involve a bunch of moving parts. Maybe a mythical SHRIKE kit to run a standard M-4 off of belted 5.56? Feed in through the ejector port and eject out through the mag well.
*edited for spelling