As I can see from data files: First: Developers make Wargot machineguns, some tech for them and make them usable. Second: They has read technical detals of "Wargot" caliber 14.5X105mm(low possibility), or has some proplems with creating special animation/configs ets for human handling (high possibility) and make this weapons
This is not M16, or M4 ... this is Colt SMG. It's looks like M16 series rifles but uses 9mm ammo. If you want more information of this design look here.