Caught me before my edit:
It's more like an M16A2, except with a sliding stock.

(Second one down)
It SHOULD be an M4, but they forgeted up the barrel portion. The M4 has tactical rails (unless I'm mistaken and the round portion comes off...?).
But what the hell; Since when do M4 Carbine's even have that rounded M16A2/AR-15 barrel cover? I was pretty sure they only came in SOPMOD or rail systems.