The G36 XM8 are both more expensive to manufacture than either the M4 or M16, and were turned down by the military because of this. But this was despite the fact that the G36 XM8 both displayed far superior performance than any AR15 design.
IMO the military should cough up the extra money and buy a rifle following the design of the G36 and chamber it in the 6.8mm round. That is what our boys need.
Some reading on the 6.8mm SPC round:
6.8 mm Remington SPC - Wikipedia, the free encyclopedia