There is an interesting article at Human Events about the US Army culture favoring the M4, while the Marines culture being pro-M16A4.
Two years ago when I was in Iraq, I noticed there were essentially two different primary infantry weapons (the M16 automatic rifle and the also-automatic M4 carbine) carried by America’s two primary ground forces — the U.S. Marine Corps and the U.S. Army.
Marines for the most part were carrying the M16. The Army on the other hand was primarily carrying the M4: a shorter, lighter version of the M16 with a collapsible-stock.
Hat Tip: Say Uncle