And going with a Bullpup, which would allow for a longer barrel in a shorter weapon would not hurt either.

A monograph by Major Thomas P. Ehrhart (PDF) is raising a bit of a stir in the blogosphere, and mostly this is used as a stopping off point for the suggestion that the 5.56mm round be replaced with something bigger and heavier.

I think that looking at the abstract gives a good picture of the problem:

Operations in Afghanistan frequently require United States ground forces to engage and destroy the enemy at ranges beyond 300 meters. While the infantryman is ideally suited for combat in Afghanistan, his current weapons, doctrine, and marksmanship training do not provide a precise, lethal fire capability to 500 meters and are therefore inappropriate. Comments from returning soldiers reveal that about fifty percent of engagements occur past 300 meters. Current equipment, training, and doctrine are optimized for engagements under 300 meters and on level terrain. This monograph reviews the small arms capability of the infantry squad from World War I to present. It then discusses current shortfalls with cartridge lethality, weapons and optics configurations, the squad designated marksman concept and finally the rifle qualification course. Potential solutions in each of these areas are discussed.

(emphasis mine)

So, the current standard weapon, the M4 is inadequate beyond 300m, and somewhat marginal at lesser distances, but the M-16 serves ably in this role:

In the table of organization for a light infantry company only the six –M240B 7.62-mm machineguns, two– 60-mm mortars and nine designated marksman armed with either 7.62-mm M14 rifles or accurized 5.56-mm M16A4’s rifles are able to effectively engage the enemy. These weapons systems represent 19 percent of the company’s firepower. This means that 81 percent of the company has little effect on the fight. This is unacceptable.

There are two reasons for this, the M4 has a 14½ inch barrel, and as velocity of the round drops, so does stopping power. The M16, with its 20 inch barrel can effectively engage at the longer distances.

The second problem is that the US Army is does not place sufficient emphasis on marksmanship:

The U.S entered World War I with a small professional Army trained in marksmanship. It filled its ranks with volunteers and conscripts and traditional marksmanship training took too much time. Between World War I and the end of the Vietnam War, the U.S. Army was a conscript Army that relied on suppressive fire, not marksmanship, and trained accordingly. The problem is suppressive fire does not do well with a light, barely lethal bullet at the distances of engagements in Afghanistan.

Note again that the bullet is “barely lethal” when fired from a 14½ inch barrel.

Most of the other western armies have moved to Bullpup, and additionally use a gas driven piston to operate the bolt, as opposed to the gas tube system used on the AR-15 family of weapons, which is sensitive to the accumulation of grime and combustion residues.

While changing the weapon and training is a non-trivial course, it is far more straightforward than a adopting a new round, particularly when there would still be issues with accuracy under the current doctrine, particularly when it is likely that our NATO allies would be disinclined to make the change in a timely manner: It took nearly a decade for the 5.56 to be adopted as NATO standard, and a decade after that for it to become the standard round for infantry weapons.

H/t Defense Tech.