M4 5.56mm Carbine

The M4 5.56mm carbine is a lightweight, gas operated, air cooled, magazine fed, selective rate, shoulder fired weapon with a collapsible stock. Equipped with a shorter barrel, collapsible stock, and subsequently accessory rails, it provides soldiers operating in close quarters with improved handling and the capability to rapidly and accurately engage targets at extended range, day or night. A shortened variant of the M16A2 rifle, the M4 provides the individual soldier operating in close quarters the capability to engage targets at extended range with accurate, lethal fire. The M4 Carbine achieves over 80 percent commonality with the M16A2 Rifle and was initially to replace all M3A1 .45 caliber submachine guns and selected M9 pistols and M16 series rifle in service. In 2005, it became the standard issue infantry weapon in the US Army.

After the military conflicts in Panama, the Persian Gulf and Somalia, the need for a shorter version of the M16A2 again appeared. While various short barreled versions of the M16 series had existed in the US military before this, they were mostly limited to special operations forces. Soon after the introduction of the M16A2, Colt engineers had begun work on a carbine variant. The offiical specifications for the XM4 were issued in 1987. For the new weapon, the barrel of the M16A2 was shortened to 14.5 inches, a length which had been found on earlier commerical Colt carbines based on the M16A1. The Colt Model 653 had been in use by special operations forces already. The fiberlite collapsible, sliding buttstock used on the Model 653 was also carried over.

The desire to mount the M203 grenade launcher on the shortened weapon required a modification of the barrel's profile. The standard M203 mount was designed to attach to the thinner profile of the M16A1, one of the reasons why this profile was maintained for the M16A2 behind the front sight block. The shortened nature of the XM4 required that the front portion of the M203's mount attach to the barrel in front of the forward sight block. A number of profiles were tested, and the final design incorporated a "step" in the barrel with a thinner profile, to allow the mounting of the launcher. In this form the weapon was type designated standard as the M4.

In 1992, Colt had introduced an upper reciever for the AR-15/M16 series that featured an accessory rail built to military specifications (MIL-STD-1913). This feature, plus a fully-automatic function, were desired by the special operations community, which had already been using fully-automatic AR-15/M16 type carbines, such as the Model 653. The US Navy had also purchased a number of weapons fitting the basic description of the XM4 in 1988 commercially from Colt. These weapons were also known by their commercial model number as Model 727s. The variant of the M4 with these features initially given the designation M4E1.

In August 1994, both variations were adopted as standard by the US Army, with the M4E1 becoming the M4A1. The first lot of M4s delivered to the US Army featured the same carry handle and sight arrangement as the M16A2. The accessory rail equipped "flat top" upper reciever found on the M4A1 was subsequently subtituted, becoming a common feature between the 2 weapons. The US Navy retained their stock of Model 727s, which were visually identical to the first M4s, but were marked with Colt's commerical nomenclature. Also in 1994, the US Army accepted the use of a heavier recoil buffer assembly to allow more reliable functioning in the shorter weapon. The US Army had initially resisted the use of the heavier buffer, hoping that it would be able to retain additional commonality with existing M16A2 rifles.

The M4 carbine was similar in design and functioning to the M16 family of rifles, thereby greatly simplifying training, supply, and maintenance. Compared to the M16A2 rifle, the M4 carbine was 1.3 pounds lighter, 6 and 5/8 inches shorter with buttstock extended, and almost 10 inches shorter with the buttstock collapsed. The original basis of issue plan was to replace, on a one-for-one basis, all caliber .45 submachine guns, selected caliber .45 and 9mm pistols, and selected Ml6A2 rifles. Infantry personnel receiving the M4 included platoon leaders, platoon sergeants, radio-telephone operators, and mortar gunners. The pistols carried by infantry commanders, executive officers, and operations officers would not be replaced.

The M4A1 carbine, differing from the M4 essentially only in the replacement of the 3 round burst with a fully-automatic fire capability, was originally intended for use by US Army Special Forces Groups, Rangers, and Special Operations Forces Aviation Units. The M4A1 would replace M4 carbine and the M16 rifle in the 75th Ranger Regiment and Special Forces Groups. The M4A1 would also replace the M4 carbine issued to flight crew members in special operations forces aviation units.

Following the introduction of the M16A4 and the Modular Weapon System (MWS) concept, a similar move was made to acquire a rail accessory handguard for US Army carbines. The carbine length version of the Knights Armament Company's (KAC) Rail Adapter System (RAS), the M5 RAS, was eventually selected. The M4 RAS was used on the M16A4 MWS. The usage of the designation M4E2 was sometimes used to describe carbines fitted with the M5 RAS, but eventually the new handguard became a standard option for existing M4 type carbines. These weapons became known as M4 or M4A1 MWS carbines.

As early as 2001, special operations forces identified a need for a heavier barrel for the M4A1 carbine to prevent catastrophic failures during sustained fire. A new profile barrel, with a greatly increased diameter between the weapon's reciever and front sight block, was subsequently provided to USSOCOM. Weapons fitted with the new barrels remained designated as M4A1s. Colt's internal nomenclature identified the standard M4A1 as the Model 921, and the subvariant with the heavier barrel as the 921HB.

In 2002, the US Army's Project Manager for Soldier Weapons issued an urgent requirement for the development of a lightweight carbine, to be designated as the XM8. The US Army's Objective Individual Combat Weapon (OICW) contract was subsequently modified to include this weapon. The prime candidate was derived from the kinetic energy component of the OICW. In 2003, this was further ammended to expand the XM8 program into a multi-component weapon system, which included rifle and carbine elements. What became the XM8 Modular Assault Weapon System was expected to cost about the same as the M4, but fielding was initially expected to be unlikely to begin before FY08. The system was also known as OICW Increment I.

Also, by the early 2000s, one of the Army National Guard's central readiness requirements was individual weapons modernization. By that time, the Army National Guard still had some 34,951 obsolete M16A1's in its inventory. Obtaining ammunition for these rifles became increasingly difficult, particularly for deployed units. While the ammunition designed for the M16A1 could be used in the M16A2 and its derivatives, the heavier ammunition used primarily in the M16A2 suffered serious performance degradation when used in the M16A1. The M16A2 and M16A4 were also being fielded to replace the first generation rifle and used the same heavier ammunition as the M4 carbine.

The M4 was issued to units and personnel with a requirement for an effective but compact, highly portable/slingable "hands free" weapon. It was ideally suited for use in close quarters and/or by soldiers who operated from vehicles with limited stowage space. The M4 could mount the optics and lighting components of the Modular Weapons System (MWS) giving it significant additional capabilities. It had become the weapon of choice for the Global War on Terror and homeland security. Any M4 carbines purchased for priority units already modernized with M16A2/A4's were to cascade those weapons to replace the much older M16A1's in other Army National Guard units.

The FY06 Army National Guard requirement for M4 Carbines was 60,943 rifles at a cost of $1k each. On hand were 15,975 with Army National Guard fielding suspended IOT push the entire weapons production to units in or deploying to Iraq. 22,648 Carbine's were programmed (Modularity) for the Army National Guard, leaving a shortfall of 22,320 weapons. Excess production capacity was available after FY05 and approximately 1000 weapons per month could be delivered on a new contract.

Funding the program was expected to allow the Army National Guard to deploy and operate with maximum effectiveness on all fronts of the Global war on Terror. It would contribute to the soldiers ability to defend him/herself and, with the MWS components, significantly increased the soldiers ability to rapidly engage targets in all environments. It was essential that the M16A1's be replaced as soon as possible. The M16A2 and A4 fieldings were funded, but alone would not displace all the M16A1's. Failure to fund the M4 fielding was seen as increasing risk to the soldiers, increasing costs of pre-deployment cross-leveling, and might degrade the Army National Guard's ability to train for and execute its federal and state missions.

In 2007, Army testing laboratories at Aberdeen Proving Ground, Maryland, subjected the M4 carbine and 3 other weapons to a severe environmental test called the "Carbine Extreme Dust Test." The lab environment allowed engineers to push the weapons beyond their technical limits to better inform and understand what was required for the most capable weapons on the current battlefield. At the time it was the third such test for the Army's carbine of choice, the M4. The other 3 weapons tested were the XM8, the Heckler and Koch 416, and the FN SCAR-L.

The Army noted all the weapons in the test performed well. The number of stoppages all the carbines exhibited was roughly one percent or less of the total rounds fired by each, meaning the weapons had over a 98 percent reliability rate under the unique conditions. Though the M4 performed exceptionally well, it came in fourth compared to the other 3 carbines in this particular extreme single-environment (dust as the only condition) testing.

The Army subsequently submitted a request for funds in the FY10 budget to examine potential replacements for the M4 carbine. Despite plans to field the XM8 carbine as a replacement, no such replacement had been adopted in the regular US Army by the end of 2009. In December 2010, it was reported that the US Army was preparing to begin the trials.

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