And with regards to fragmentation of non exotic 5.56, this from the ammo-oracle (a 1 Gb download pdf)

Temperature, altitude and humidity are other factors. As temperature or altitude increases, air becomes less dense and bullets travel faster. Contrary to common conceptions, as humidity increases air also becomes less dense and helps bullets retain velocity.
It is important, then, to keep in mind that any statistics given can only be approximate and can be affected by a wide range of factors. But as a baseline, these numbers are what you could expect for 75° F, 25% humidity, at sea level, from various barrel lengths:

Distance to 2700 fps for M193
20" Barrel 190-200m
16" Barrel 140-150m
14.5" Barrel 95-100m
11.5" Barrel 40-45m

Distance to 2700 fps for M855:
20" Barrel 140-150m
16" Barrel 90-95m
14.5" Barrel 45-50m
11.5" Barrel 12-15m

And:

Interesting, few of these reports [stopping power] seem to be coming from troops 20" or SAW platforms. It would seem that the additional velocity from the longer barrel provides adequate usable fragmentation range for M855 in the majority of cases. From shorter barrels, such as the M4's 14.5" barrel, M855's fragmentation range varies from as much as 90m to as little as 10m, which frequently isn't enough range.