The Army dust chamber tests designed to see how the M4 carbine performs against other carbines has been postponed until December, four months after its originally planned August start.

The upcoming tests will pit the M4 against the Heckler & Koch 416, the H&K XM8 and FNH USA's Special Operations Forces Combat Assault Rifle, also known as SCAR.

The SCAR sample models, which are still in development, will not be delivered until December, Brig. Gen. Mark Brown, the commander of Program Executive Office Soldier, told Army Times recently. Brown said the tests will begin when all 10 sample models of each weapon are present. The test will feature weapons officials shooting 6,000 rounds though each weapon under sandstorm conditions.

Weapons officials from PEO Soldier scheduled the tests to be performed at Army Test and Evaluation Center at Aberdeen Proving Ground, Md., at the request of Sen. Tom Coburn, R-Okla., in July. Coburn began questioning the Army more than four months ago about its plans to spend $375 million to purchase M4s through fiscal 2009. Lighter and more compact than the M16 rifle, the M4 is more effective for the close confines of urban combat. The Army began fielding the M4 in the mid-1990s.

Coburn questioned the M4's "long-standing reliability" problems in his original April 12 letter and asked if the Army had considered newer, possibly better weapons available on the commercial market.

Army weapons officials at Fort Benning's Infantry Center in Georgia — the command responsible for determining soldiers' weapons needs — maintain that the M4 carbine meets the Army's requirements and see no reason to replace it.

Brown said he received no complaints about the M4 carbine’s performance from soldiers during a recent trip to Iraq.

The contenders participating in the upcoming test use a piston-style operating system, which relies on a gas-driven piston rod to cycle the weapon during firing.

By contrast, the M4 uses a gas tube system, which relies on the gas created when a bullet is fired to cycle the weapon. Weapons experts said blowing gas directly into the receiver of the weapon spews carbon residue that can lead to fouling and heat that dries up lubrication and causes excessive wear on parts.

The Army's Delta Force replaced its M4s with the H&K 416 in 2004. The elite unit collaborated with the German arms maker to develop the new carbine. Experts said its piston operating system significantly reduces malfunctions while increasing the life of parts. Other special units, such as the Army's Asymmetric Warfare Group, also have used the 416.

U.S. Special Operations Command has also revised its small-arms requirements. In November 2004, SOCom awarded a developmental contract to FN Herstal to develop its new SCAR to replace its weapons from the M16 family.

And from 2002 to 2005, the Army developed the XM8 as a replacement for the conventional Army's M16 family. The program led to infighting in the service's weapons community and eventually died after failing to win approval at the Defense Department level.

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