The 6.8 SPC and, even better, the 6.5 Grendel use the same mags, lowers, action, in fact, everything but the barrel assembly. The 6.5 is way better than the 5/56, and so good, that in excess of 300 meters, it's better than the 7.62!!!
ASSAULT RIFLES AND THEIR AMMUNITION: HISTORY AND PROSPECTS
THE .256" BRITISH: A LOST OPPORTUNITY
Ammunition Improvements for 21st Century Mil/LE Urban Operations
I'd prefer the 6.8 or 6.5 myself. The M16, IMO, isn't a bad REMF toy, but for combat arms soliders and Marines, they need something with some knockdown power.
The helmets suck. Suck. suck. suck. Consdidering how much they weight, they don't even stop a 9mm round. Wearing one for long periods blows. My unit did get decent boots when we were in Kuwait. The Belleville waffle-tread ones are terrible. The Marines who came back from Iraq, some of them would have paper-thin soles on their boots. The official goggles suck too. Suck. suck. suck. Also, instead of BCGs, the military should just issue prescription Wiley's. Those are great.