Well i think the army is looking to replace it with the XM8 but i thought it was more M4 size then M16. If i was us military and was looking to replace like with like i would want the G36, as it is 5.56mm, used by its NATO allies and is one of the best rifles in the world at the minute, though it isnt bullpup design!
http://www.world.guns.ru/assault/as14-e.htm