The range difference between the MEA and USMC is largely because the USMC has the Future Warrior system of integrated red-dot sights, thermal imaging and weapon mounted camera.
As for the range difference between rifles and carbines the effect would be minimal at the ranges depicted in the game. If ,on the other hand, you're talking about relistic ranges the M4 and M16 would shoot about the same size group at 150 yards or 75 table top inches. Beyond that you would notice a difference, maybe at around 100 inches.