The KitUp blog quotes Commandant of the Marine Corps, Gen. James Conway …

Marines like that M4 carbine because it looks cool. And I’ve had some Marines complain to the chairman of the Joint Chiefs of Staff saying ‘you know, the officers are getting these things, but we’re still having to carry this rifle.’ Well, the Marine Corps will always be a rifle Marine Corps. The carbine is an extension of the pistol, not a reduction of a rifle. And in the Afghanistan scenario where you’re shooting long distances you gotta be able to reach out and touch ‘em. And a carbine is just not designed to do that.

M16A4 rifles

The difference between a rifle and a carbine is constantly changing. The Soviet Mosin Nagant Model 1944 (M44) Carbine, for example, had a barrel slightly longer than the M16A4 rifle has (20.2″ vs. 20″). Today, many think that 18″ is considered be the current maximum length of a carbine, although I personally think 16″ is a more modern definition. I believe Conway is shortsighted if he thinks that the USMC will never adopt a 18″, or shorter, barreled weapon.