They are I guess doing this to ostensibly increase the range to jump up from 400 to 650 meters. They could have just put a 20 inch barrel on the M4.
How is that going to more than double the weight of the bullet?
The 5.56 has been a marginal round from the beginning and it's inadequacy showed itself even more when we started fighting in the desert with it's increased ranges. It's long overdue that the military deployed a better round.