And it says it is open to replacing it with a higher caliber weapon, highlighting long-standing criticism of the existing 5.56mm round.

In August, the Army posted a "Request For Information" -- a procurement document that asks the industry what it can offer -- on a possible successor to the M4, manufactured by West Hartford, Conn.-based Colt Defense LLC. The military has looked at alternatives in the past, and some elite Special Forces units have already retired the M4 in favor of newer, higher-caliber weapons.

On Nov. 13, the Army will hold an industry day to examine the full range of state-of-the-art weapons on the market today. The event also will allow contractors to "more freely address the questions that were posed in the request for information," said Richard Audette, deputy project manager for PEO Soldier Weapons, the unit of the Army responsible for weapons procurement.

At Fort Benning, Ga., the U.S. Army Infantry School is drafting a new set of requirements, expected to be published in May or June, which will spell out what the branch is looking for in a new carbine.

After the arms industry has a better idea what the Army wants, companies will be able to start pitching their weapons in an open competition, tentatively scheduled to begin next summer.

The Request For Information is also notable because, for the first time, the Army says it is open to any caliber of weapon, greatly expanding the number of guns available for competition.

The M16 and M4 use the 5.56mm round, which is smaller and, some complain, has less stopping power than the 7.62mm round used by weapons such as the AK-47.

Though open to the possibility of a new caliber weapon, officials at PEO Soldier said that a change from the 5.56mm round would have to demonstrate a "revolutionary operational benefit." The Infantry School's requirement will nail down exactly what a revolutionary benefit means, but Audette said the military produces a billion rounds of 5.56mm ammunition a year, so the benefit would have to be great enough to offset the cost of changing such a huge level of production.

The Army won't release the names of companies who plan to attend the industry day, but representatives from Colt, Barrett, Heckler Koch and FN-Herstel have all said they will take part.

The sheer size of the Army's purchasing power makes any potential change to its standard firearm a big deal. The service purchases more than 100,000 M4s a year at a cost of $178 million, and has 400,000 in service.

The vast majority of soldiers -- 92 percent in a recent survey -- say they are content with the M4, a carbine, essentially a shorter version of the M16 rifle. Its truncated barrel provides greater maneuverability in close quarters combat such as the urban environments soldiers face today in places like Baghdad, and the smaller round means spare ammunition is smaller and lighter. So why the need to change it?

The weapon, in use for more than 40 years, has a number of problems, which are addressed by newer guns. The M4 tends to jam at a much higher rate than newer weapons -- in a 2007 dust test, the weapon jammed at four to seven times the rate of the competition.

Colt, manufacturer of the M4, says if the weapons are meticulously maintained and cleaned on a regular basis, jams can be kept to a minimum. However, many of the new firearms operating systems require less maintenance to achieve the same effect.

The new requirements will offer a full list of the M4's shortcomings, but right now the Army unit PEO Soldier is looking for increased accuracy and increased reliability as key points to address.

Complicating things for Colt, the technical package for the M4 becomes government property, meaning the Army can license another gunmaker to produce the weapon for less, though the military may end up renegotiating with Colt. Since it will take at least two to three years to start buying replacements for the M4 and a decade before the weapon is phased out, the fate of the M4 contract remains an important point.

In addition to replacing the M4, the Army also might be looking to change its main subcompact weapon -- essentially a really short machine gun -- which is used mostly by aircraft pilots who need a high degree of maneuverability inside a cockpit.

Many of the newer weapons are modular systems, meaning that a single weapon can change into a few different guns by adding a shorter or longer barrel and removing the buttstock.

Barbara Muldowney, deputy manager for individual weapons at PEO Soldier, said a modular system is a selling point from the Army's perspective.

"If we can find a weapon that could do two things with a few changes, then obviously we're getting a two for one," she said.

One long-touted alternative to the M4 is the XM8, developed by the German gunmaker Heckler Koch, which worked closely with the Army on developing that weapon.

The plan to use the XM8 as a replacement for the M4 was canceled in 2005 amid a welter of charges and countercharges. Executives at Heckler Koch blamed procurement politics, but others said it was put on hold because the Army lacked a requirement for a new weapon at the time. Colt claimed the new weapon offered no significant advantages.

Now that it is shopping around for something new, the Army is open to the XM8. However, Audette said it is up to HK to make the first move, something the company said it plans to do.

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