After nearly 40 years of battlefield service around the globe, the M-16 (search) may be on its way out as the standard Army assault rifle because of flaws highlighted during the invasion and occupation of Iraq.

U.S. officers in Iraq say the M-16A2 — the latest incarnation of the 5.56 mm firearm — is quietly being phased out of front-line service because it has proven too bulky for use inside the Humvees and armored vehicles that have emerged as the principal mode of conducting patrols since the end of major fighting on May 1.

The M-16, at nearly 40 inches, is widely considered too long to aim quickly within the confines of a vehicle during a firefights, when reaction time is a matter of life and death.

"It's a little too big for getting in and out of vehicles," said Brig. Gen. Martin Dempsey, commander of the 1st Armored Division (search), which controls Baghdad. "I can tell you that as a result of this experience, the Army will look very carefully at how it performed."

Instead of the M-16, which also is prone to jamming in Iraq's dusty environment, M-4 carbines (search) are now widely issued to American troops.

The M-4 is essentially a shortened M-16A2, with a clipped barrel, partially retractable stock and a trigger mechanism modified to fire full-auto instead of three-shots bursts. It was first introduced as a personal defense weapon for clerks, drivers and other non-combat troops.

"Then it was adopted by the Special Forces and Rangers, mainly because of its shorter length," said Col. Kurt Fuller, a battalion commander in Iraq and an authority on firearms.

Fuller said studies showed that most of the combat in Iraq has been in urban environments and that 95 percent of all engagements have occurred at ranges shorter than 100 yards, where the M-4, at just over 30 inches long, works best.

Still, experience has shown the carbines also have deficiencies. The cut-down barrel results in lower bullet velocities, decreasing its range. It also tends to rapidly overheat and the firing system, which works under greater pressures created by the gases of detonating ammunition, puts more stress on moving parts, hurting its reliability.

Consequently, the M-4 is an unlikely candidate for the rearming of the U.S. Army. It is now viewed as an interim solution until the introduction of a more advanced design known as the Objective Individual Combat Weapon (search), or OICW.

There is no date set for the entry into service of the OICW, but officers in Iraq say they expect its arrival sooner than previously expected because of the problems with the M-16 and the M-4.

"Iraq is the final nail in the coffin for the M-16," said a commander who asked not to be identified.

The current version of the M-16 is a far cry from the original, which troops during the Vietnam War criticized as fragile, lacking power and range, and only moderately accurate. At the time, a leading U.S. weapons expert even recommended that American soldiers discard their M-16s and arm themselves with the Kalashnikov AK-47 (search) rifle used by their Vietcong enemy.

Although the M16A1 — introduced in the early 1980s — has been heavily modernized, experts say it still isn't as reliable as the AK-47 or its younger cousin, the AK-74. Both are said to have better "knockdown" power and can take more of a beating on the battlefield.