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M4 carbine

Carbine, 5.56 mm, M4
Colt M4 Carbine with ACOG Scope
Type Carbine, assault rifle
Place of origin United States
Service history
In service 1994–present
Used by See Users below
Production history
Unit cost $700[3]
Produced 1994–present
Variants M4A1, CQBR (Mk. 18)
Weight 6.36 lb (2.88 kg) empty
7.5 lb (3.4 kg) with 30 rounds
Length 33 in (840 mm) (stock extended)
29.75 in (756 mm) (stock retracted)
Barrel length 14.5 in (370 mm)

Cartridge 5.56×45mm NATO
Caliber 5.56 mm (.223 in)
Barrels 1
Action Gas-operated, rotating bolt (Direct impingement)
Rate of fire 700–950 round/min cyclic[4]
Muzzle velocity 2,900 ft/s (880 m/s)[5]
Effective firing range 500 m (550 yd)[6]
Feed system 30-round box magazine or other STANAG magazines. Other magazines with different capacities also available.
Sights Iron sights or various optics

The M4 carbine is a shorter and lighter variant of the M16A2 assault rifle. The M4 is a 5.56×45mm NATO, air-cooled, direct impingement gas-operated, magazine-fed carbine. It has a 14.5 in (370 mm) barrel and a telescoping stock. The M4 carbine is heavily used by the United States Armed Forces and is replacing the M16 rifle in most United States Army combat units.[7]

The M4 is also capable of mounting the M203 grenade launcher. The distinctive step in their barrel is for mounting the M203 with the standard hardware. The M4 is capable of firing in semi-automatic and three-round burst (like the M16A2 and M16A4), while the M4A1 is capable of firing in semi-auto and full automatic (like the M16A1 and M16A3).


  • History 1
    • Improved M4 1.1
      • Army upgrades 1.1.1
  • Design 2
    • Accessories 2.1
      • Feedramps 2.1.1
      • SOPMOD Block I 2.1.2
      • SOPMOD Block II 2.1.3
  • Variants 3
    • M4 MWS (Modular Weapon System) 3.1
    • M4A1 3.2
    • Mark 18 CQBR 3.3
    • Enhanced M4 3.4
    • M4 Commando 3.5
    • Armwest LLC M4 3.6
  • Performance 4
    • Early feedback 4.1
    • 2006 CNA report 4.2
    • 2007 dust test 4.3
    • Reliability 4.4
    • Gas piston 4.5
  • Trademark issues 5
  • Users 6
    • U.S. civilian ownership 6.1
  • See also 7
    • Replacement attempts 7.1
  • References 8
  • External links 9


Following the adoption of the M16 rifle, carbine variants were also adopted for close quarters operations. The CAR-15 family of weapons served through the Vietnam War. However, these carbines had design issues, as "the barrel length was halved" to 10 inches which "upset the ballistics", reducing its range and accuracy and "led to considerable muzzle flash and blast, so that a large flash suppressor had to be fitted".[8] "Nevertheless, as a short-range weapon it is quite adequate and thus, (despite) its caliber, (the XM177 "Commando") is classed as a submachine gun."[8] In 1988, Colt began work on a new carbine design called the XM4 combining the best features of the Colt Commando and M16A2 rifles.

The XM4 was given a longer 14.5-inch barrel with the M16A2's 1:7 inch rifle twist, to use the heavier 62-grain M855 rounds. The extended barrel improved the XM4's ballistics, reduced muzzle blast and gave the XM4 the ability to mount a bayonet and the M203 grenade launcher. The XM4 was also given the M16A2's improved rear sight and cartridge deflector, as well as other minor refinements. In 1994, the U.S. military officially accepted the XM4 into service as the M4 carbine to replace M16A2s in certain roles.[9] The M4 carbine has also replaced most submachine guns and selected handguns in U.S. military service,[9] as it fires more effective rifle ammunition that offers superior stopping power and is better able to penetrate modern body armor.

The United States Marine Corps has ordered its officers (up to the rank of Lieutenant Colonel) and staff non-commissioned officers to carry the M4 carbine instead of the M9 handgun.[10] This is in keeping with the Marine Corps doctrine, "Every Marine a rifleman." The Marine Corps, however, chose the full-sized M16A4 over the M4 as its standard infantry rifle. United States Navy corpsmen E5 and below are also issued M4s instead of the M9.[11] While ordinary riflemen in the Marine Corps are armed with M16A4s, M4s are fielded by troops in positions where a full-length rifle would be too bulky, including vehicle operators and fireteam and squad leaders. As of 2013, the U.S. Marine Corps has 80,000 M4 carbines in their inventory.[12][13] However, by July 2015 major Marine Corps commands were endorsing switching to the M4 over the M16A4 as the standard infantry rifle, just as the Army had done. This is because of the carbine's lighter weight, compact length, and addressing modern combat situations that mostly happen within close quarters; if a squad needs to engage at longer ranges, the M27 IAR can be used as a designated marksman rifle. Approval of the change would move the M16 to support personnel, while armories already have the 17,000 M4s in the inventory needed to outfit all infantrymen who need one.[14] In October 2015, Commandant Robert Neller formally approved of making the M4 carbine the primary weapon for all infantry battalions, security forces, and supporting schools in the U.S. Marine Corps. The switch is to begin in early 2016 and be completed by September 2016.[15]

Improved M4

On 1 July 2009, the U.S. Army took complete ownership of the M4 design.[16] This allowed companies other than Colt to compete with their own M4 designs. The Army planned on fielding the last of its M4 requirement in 2010.[16] On 30 October 2009, Army weapons officials proposed a series of changes to the M4 to Congress. Requested changes included an electronic round counter that records the number of shots fired, a heavier barrel, and possibly replacing the direct impingement system with a gas piston system.

The benefits of this, however, have come under scrutiny from both the military and civilian firearms community.[17][18] According to a PDF detailing the M4 Carbine improvement plans released by PEO Soldier, the direct impingement system would only be replaced after reviews were done comparing the direct impingement system to commercial gas piston operating system to find out and use the best available operating system in the U.S. Army's improved M4A1.[19]

In September 2010, the Army announced it would buy 12,000 M4A1s from Colt Firearms by the end of 2010, and would order 25,000 more M4A1s by early 2011. The service branch planned to buy 12,000 M4A1 conversion kits in early 2011. In late 2011 the Army bought 65,000 more conversion kits. From there the Army had to decide if it would upgrade all of its M4s.[20]

On 21 April 2012, the U.S. Army announced to begin purchasing over 120,000 M4A1 carbines to start reequipping front line units from the original M4 to the new M4A1 version. The first 24,000 were to be made by Remington Arms Company. Remington was to produce the M4A1s from mid-2013 to mid-2014.[21] After completion of that contract, it was to be between Colt and Remington to produce over 100,000 more M4A1s for the U.S. Army. Because of efforts from Colt to sue the Army to force them not to use Remington to produce M4s, the Army reworked the original solicitation for new M4A1s to avoid legal issues from Colt.[22] On 16 November 2012, Colt's protest of Remington receiving the M4A1 production contract was dismissed.[23] Instead of the contract being re-awarded to Remington, the Army awarded the contract for 120,000 M4A1 carbines worth $77 million to FN Herstal on 22 February 2013.[24][25] The order is expected to be completed by 2018.[26]

Army upgrades

The M4 product improvement program (PIP) is the effort by the U.S. Army to modernize its fleet of M4 service rifles. Phase I consists of converting and replacing regular M4s with the M4A1 version. This variant of the rifle is fully automatic and has a heavier barrel, and is given ambidextrous fire controls. Phase II of the PIP explored developing a new bolt carrier. 11 designs were submitted. The competition was scheduled to conclude in summer 2013, but ended in April 2012. Over six months of testing revealed that the current bolt carrier assembly outperformed the competing designs, especially in the areas of reliability, durability, and high-temp and low-temp tests. Phase II also includes a competition for a free-floating forward rail assembly. The Army may award contracts to up to three finalists in early 2013, with the selection of a final winner in early 2014. If the Army determines that the winning rail system should be procured, delivery of new rail is anticipated by the summer of 2014.[27]

In March 2015, the Army launched a market survey to see what the small-arms industry could offer to further enhance the M4A1 to an "M4A1+" standard. Several upgrade options include an extended forward rail that will allow for a free-floated barrel for improved accuracy with a low-profile gas block that would do away with the traditional triangular fixed front sight, removable front and rear flip-up back-up iron sights, a coyote tan or "neutral color" rail for reduced visual detection, a more effective flash suppressor/muzzle brake, an improved charging handle, and a new single-stage trigger module.[28]


M4 with M68 Close Combat Optic and AN/PAQ-4

The M4 and its variants fire 5.56×45mm NATO ammunition (and .223 Remington ammunition) and are gas-operated, magazine-fed, selective fire firearms with either a multi-position telescoping stock or a fixed A2 or LE tactical stock.[29]

The M4 is a shorter and lighter variant of the M16A2 rifle, with 80% parts commonality.[30] The 20% of the parts that are not interchangeable include the buffer spring and weight, barrel, and gas tube due to the shorter carbine length. Original M4 models had a flat-ended telescoping stock, but newer models are now equipped with a redesigned telescoping stock that is slightly larger with curvature at the end.[31] The M4 is similar to much earlier compact M16 versions, such as the 1960s-era XM177 family. Some of those visual designs are obvious in both weapons.

The M4 with the newer, redesigned telescoping stock

As with many carbines, the M4 is handy and more convenient to carry than a full-length rifle. The price is slightly inferior ballistic performance compared to the full-size M16, with its 5.5" (14 cm) longer barrel. This becomes most apparent at ranges of 200 yards and beyond.

While the M4's maneuverability makes it a candidate for non-infantry troops (vehicle crews, clerks and staff officers), it also makes it ideal for close quarters battle (CQB). The M4, along with the M16A4, have mostly replaced the M16A2 in the Army and Marines. The U.S. Air Force, for example, has transitioned completely to the M4 for Security Forces squadrons, while other armed personnel retain the M16A2. The US Navy uses M4A1s for Special Operations and by vehicle crews.

Some features of the M4 and M4A1 compared to a full-length M16-series rifle include:

  • Compact size
  • Shortened barrel 14.5 in (370 mm), which includes the shorter carbine gas system.
  • Telescoping buttstock

However, there have been some criticisms of the carbine, such as lower muzzle velocities and louder report due to the shorter barrel, additional stress on parts because of the shorter gas system, and a tendency to overheat faster than the M16A2.


An M4A1 just after firing, with an ejected case in mid-air; the M203 and M68 CCO are attached.

Like all the variants of the M16, the M4 and the M4A1 can be fitted with many accessories, such as night vision devices, suppressors, laser pointers, telescopic sights, bipods, either the M203 or M320 grenade launchers, the M26 MASS shotgun, forward hand grips, and anything else compatible with a MIL-STD-1913 Picatinny rail.

Other common accessories include the AN/PEQ-2, Advanced Combat Optical Gunsight (ACOG), and M68 CCO. EOTech holographic weapon sights are part of the SOPMOD II package. Visible and IR (infrared) lights of various manufacturers are also commonly attached using various mounting methods. As with all versions of the M16, the M4 accepts a blank-firing attachment (BFA) for training purposes.


M4 feedramps are extended from the barrel extension into the upper receiver. This can help alleviate feeding problems which may occur as a result of the increased pressure of the shortened gas system of the M4. This problem is primarily seen in full-auto applications.


SOPMOD (Special Operations Peculiar Modification) Block I

U.S. Special Operations Command (USSOCOM) developed the Special Operations Peculiar Modification (SOPMOD) Block I kit for the carbines used by units under its jurisdiction. The kit features an M4A1, a Rail Interface System (RIS) handguard developed by Knight's Armament Company, a shortened quick-detachable M203 grenade launcher and leaf sight, a KAC sound suppressor, a KAC back-up rear sight, an Insight Technologies AN/PEQ-2A visible laser/infrared designator, along with Trijicon's ACOG TA-01NSN model and Reflex sights, and a night vision sight. This kit was designed to be configurable (modular) for various missions, and the kit is currently in service with special operations units.


M4A1 SOPMOD Block II in Afghanistan 2012.

A second-generation SOPMOD kit (now known as SOPMOD II) includes innovative optics, such as the Elcan Specter DR, Trijicon's ACOG TA-31 ECOS model, and the Eotech 553. Block II uses the RIS II rails manufactured by Daniel Defense in both a 9.5 and 12.5 length.


Except for the very first delivery order, all U.S. military-issue M4 and M4A1 carbines possess a flat-top NATO M1913-specification (Picatinny) rail on top of the receiver for attachment of optical sights and other aiming devices — Trijicon TA01 and TA31 Advanced Combat Optical Gunsights (ACOG), EOTech 550 series holographic sights, and Aimpoint M68 Close Combat Optic (M68 CCO) being the favorite choices — and a detachable rail-mounted carrying handle. Standards are the Colt Model 920 (M4) and 921 (M4A1).

Variants of the carbine built by different manufacturers are also in service with many other foreign special forces units, such as the Australian Special Air Service Regiment (SASR). While the SASR uses weapons of essentially the same pattern built by Colt for export (Colt uses different models to separate weapons for the U.S. military and those for commercial/export purposes), the British SAS uses a variant on the basic theme, the Colt Canada (formerly Diemaco) C8SFW.

M4 MWS (Modular Weapon System)

M4 MWS (Modular Weapon System) shown with various accessories including M203 grenade launcher, RIS foregrip, removable carry handle/rear sight assembly, AN/PEQ-4 laser system, M68 CCO reflex sight, and the AN/PVS-4 night vision optics

Colt Model 925 carbines were tested and fitted with the Knight's Armament Corporation (KAC) M4 RAS under the designation M4E2, but this designation appears to have been scrapped in favor of mounting this system to existing carbines without changing the designation. The U.S. Army Field Manual specifies for the Army that adding the Rail Adapter System (RAS) turns the weapon into the M4 MWS or Modular Weapon System.


The M4A1 carbine is a fully automatic variant of the basic M4 carbine intended for special operations use. The M4A1 has a "S-1-F" (safe/semi-automatic/fully automatic) trigger group, while the M4 has a "S-1-3" (safe/semi-automatic/3-round burst) trigger group. The M4A1 is used by almost all U.S special operation units including, but not limited to, Marine Force Recon, Army Rangers, Army Special Forces, Navy SEALs, United States Air Force Pararescue and Air Force Combat Control Teams. The M4A1 is especially favored by counter-terrorist and special forces units for close quarters combat and urban warfare because of the carbine's compact firepower. It has a maximum effective range of about 500 to 600 meters (550–660 yd).[6] The fully automatic trigger gives a more consistent trigger pull, which leads to better accuracy.[32] According to Mark A. Westrom, owner of ArmaLite, Inc., automatic fire is better for clearing rooms than burst fire.[33]

In the last few years, M4A1 carbines have been refit or received straight from factory with barrels with a thicker profile under the handguard. This is for a variety of reasons such as heat dissipation during full-auto, and accuracy as a byproduct of barrel weight. These heavier barrel weapons are also fitted with a heavier buffer known as the H2. Out of three sliding weights inside the buffer, the H2 possesses two tungsten weights and one steel weight, versus the standard H buffer, which uses one tungsten weight and two steel weights. These weapons, known by Colt as the Model 921HB (for Heavy Barrel), have also been designated M4A1, and as far as the government is concerned the M4A1 represents both the 921 and 921HB.

Conversion of M4s to the M4A1 began in 2014, the start of all U.S. Army forces being equipped with the automatic variant.[34] Though in service with special forces, combat in Afghanistan showed the need for providing automatic suppression fires during fire and movement for regular soldiers. The 101st Airborne Division began fielding new-built M4A1s in 2012, and the U.S. 1st Infantry Division became the first unit to convert their M4s to M4A1-standard in May 2014. Upgrades included a heavier barrel to better dissipate heat from sustained automatic firing, which also helps the rifles use the M855A1 EPR that has higher proof pressures and puts more strain on barrels. The full-auto trigger group has a more consistent trigger pull, whereas the burst group's pull varies on where the fire control group is set, resulting in more predictable and better accuracy on semi-automatic fire. Another addition is an ambidextrous selector lever for easier use with left-handed shooters. The M4-M4A1 conversion only increases weapon weight from 7.46 lb (3.38 kg) to 7.74 lb (3.51 kg), counting a back-up iron sight, forward pistol grip, empty magazine, and sling. Each carbine upgrade costs $240 per rifle, for a total cost of $120 million for half a million conversions. 300 conversions can be done per day to equip a brigade combat team per week, with all M4A1 conversions to be completed by 2019.[35][36]

Mark 18 CQBR

An M4A1 with a Close Quarter Battle Receiver. The barrel length is 10.3 inches.

The Mk 18 Close Quarters Battle Receiver is an M4A1 with a 10.3-inch barrel upper receiver.[37] Current contractors for the Mark 18 are Colt and Lewis Machine & Tool (LMT) NSN 1005-01-527-2288.

Enhanced M4

For the Individual Carbine competition, Colt submitted their Enhanced M4 design, also known as the Colt Advanced Piston Carbine (APC). The weapon has a suppression-ready fluted barrel, which is lighter and cools better than previous M4 barrels. It is claimed to have "markedly better" accuracy. To improve reliability, Colt used an articulating link piston (ALP) which "reduces the inherent stress in the piston stroke by allowing for deflection and thermal expansion".[38] In traditional gas piston operating systems, the force of the piston striking the bolt carrier can push the bolt carrier downwards and into the wall of the buffer tube, leading to accelerated wear and even chipped metal. This is known as carrier tilt. The ALP allows the operating rod to wiggle to correct for the downward pressure on the bolt and transfers the force straight backwards in line with the bore and buffer assembly, eliminating the carrier tilt. This relieves stress on parts and helps to increase accuracy.[39] The Individual Carbine competition was canceled before a winning weapon was chosen.[32]

M4 Commando

Though Colt has focused its attention on carbines with 14.5-inch barrels and rifles with 20-inch barrels, Colt continues to make carbines with 11.5-inch barrels, which it calls Commandos. Originally, Commandos were assembled from whatever spare parts are available, so Model 733 Commandos could have A1-style upper receivers with case deflectors or A2-style upper receivers, and M16A1-profile 1:7 or M16A2-profile 1:7 barrels. Depending on the specific models, Commandos may have had three-position fire control groups (safe/semi-automatic/three-round burst), or four-position having both full-automatic and burst. The modern Model 933 has a "flattop" receiver, with a removable carrying handle and a MIL-STD-1913 Picatinny rail, with semi-automatic and automatic fire. The Model 935 Commando has the features of the Model 933, but has three-round burst fire instead of automatic. Though originally called the M16A2 Commando, Colt markets them as the M4 Commando.

Armwest LLC M4

In 2014, American firearms designer Jim Sullivan provided a video interview regarding his contributions to the M16/M4 family of rifles when working for Armalite. A noted critic of the M4 he illustrates the deficiencies found in the rifle in its current configuration. In the video, he demonstrates his "Arm West LLC modified M4", with enhancements he believes necessary to rectify the issues with the weapon. Proprietary issues aside the weapon is said to borrow features in his prior development, the Ultimax. Sullivan has stated (without exact details as to how) the weapon can fire from the closed bolt in semi-automatic and switch to open bolt when firing in fully automatic improving accuracy. The weight of the cyclic components of the gun have been doubled (while retaining the weapons weight at less than 8 pounds). Compared to the standard M4 which in automatic fires 750-950 rounds a minute, the rate of fire of the Arm West M4 is heavily reduced both to save ammunition and reduce barrel wear, the reduced rate also renders the weapon more controllable and accurate in automatic firing.[40]


Fire Controlman Seaman Rachel Hubley fires an M4 carbine from the guided-missile cruiser USS Vella Gulf.

The M4 Carbine has been used for close quarters operations where the M16 would be too long and bulky to use effectively. It has been a compact, light, customizable, and accurate weapons platform. This has come at the cost of reliability and maintainability. Failure to maintain the M4 causes malfunctions. This became apparent as it saw continued use in the sandy environments of Iraq and Afghanistan.[41] Despite this, in post-combat surveys, 94 percent of soldiers rated the M4 as an effective weapons system.[42]

Early feedback

By late 2002, 89 percent of U.S. troops reported they were confident with the M4, but they had a range of problems. 34 percent of users said the handguards rattled and became excessively hot when firing, and 15 percent had trouble zeroing the M68 Close Combat Optic. 35 percent added barber brushes and 24 percent added dental picks to their cleaning kits. There were many malfunctions, including 20 percent of users experiencing a double feed, 15 percent experiencing feeding jams, and 13 percent saying that feeding problems were due to magazines. 20 percent of users were dissatisfied with weapon maintenance. Some had trouble locking the magazine into the weapon and having to chamber a round in order to lock the magazine. Soldiers also asked for a larger round to be able to kill targets with one shot. New optics and handguards made usage of the M4 easier, and good weapon maintenance reduced the number of misfeeds.[43]

2006 CNA report

In December 2006, the Center for Naval Analyses released a report on U.S. small arms in combat. The CNA conducted surveys on 2,608 troops returning from combat in Iraq and Afghanistan over the past 12 months. Only troops who fired their weapons at enemy targets were allowed to participate. 917 troops were armed with M4 Carbines, making up 35 percent of the survey. 89 percent of M4 users (816 troops) reported they were satisfied with the weapon. 90 percent (825 troops) were satisfied with handling qualities such as handguards, size, and weight. M4 users had the highest levels of satisfaction with weapon performance, including 94 percent (862 troops) with accuracy, 92 percent (844 troops) with range, and 93 percent (853 troops) with rate of fire. Only 19 percent of M4 users (174 troops) reported a stoppage, while 82 percent of those that experienced a stoppage said it had little impact on their ability to clear the stoppage and re-engage their target. 53 percent of the M4 users (486 troops) never experienced failures of their magazines to feed. 81 percent (743 troops) did not need their rifles repaired while in theater. 80 percent (734 troops) were confident in the M4's reliability, defined as level of soldier confidence their weapon will fire without malfunction, and 83 percent (761 troops) were confident in its durability, defined as level of soldier confidence their weapon will not break or need repair. Both factors were attributed to high levels of soldiers performing their own maintenance. 54 percent of M4 users offered recommendations for improvements. 20 percent of requests were for greater bullet lethality, and 10 percent was better quality magazines, as well as other minor recommendations. Some M16 users expressed their desire to be issued the M4.[44] Some issues have been addressed with the issuing of the improved STANAG magazine in March 2009,[45][46] and the M855A1 Enhanced Performance Round in June 2010.[47]

2007 dust test

In the fall 2007, the Army tested the M4 against three other carbines in "

  • Colt M4's Law Enforcement page and Colt M4's Military page
  • US Army M4 fact file
  • The AR-15/M16 Magazine FAQ
  • U.S. Army Won't Field Rifle Deemed Superior to M4
  • Online Army Study Guide

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Replacement attempts

See also

Civilian replicas of the M4 typically have 16 inch barrels (or standard 14.5 inch M4 barrels with permanently attached flash suppressors to bring the effective length to 16 inches) and are semi-automatic only to meet the legal definition of a rifle under Title I (Gun Control Act). The M4 falls under restrictions of Title II (National Firearms Act): the 14.5 inch barrel makes the M4 a Short Barrel Rifle (SBR) and select fire capability (semi-automatic and full automatic or burst-automatic) makes the M4 a machinegun. Civilian-legal M4s are also popular with police as a patrol carbine.

Sales of select-fire or full automatic M4s by Colt are restricted to military and law enforcement agencies. Only under special circumstances can a private citizen own an M4 in a select-fire or fully automatic configuration. While many machine guns can be legally owned with a proper tax stamp from the Bureau of Alcohol, Tobacco, Firearms and Explosives, an amendment to the Firearm Owners Protection Act of 1986 barred the transfer to private citizens of machine guns made or registered in the U.S. after May 19, 1986. The only exception was for Special Occupational Taxpayers (SOT): licensed machine gun dealers with demonstration letters, manufacturers, and those dealing in exports and imports. As such, only the earliest Colt M4 prototypes built prior to May 19, 1986 would be legal to own by civilians not in the categories mentioned. The modular nature of the AR-15 design, however, makes it a relatively simple matter to fit M4-specific components to a "pre-'86" select-fire AR-15 lower receiver, producing an "M4" in all but name.

U.S. civilian ownership


The M4 was developed and produced for the Heckler & Koch and Bushmaster Firearms, claiming acts of trademark infringement, trade dress infringement, trademark dilution, false designation of origin, false advertising, patent infringement, unfair competition, and deceptive trade practices. Heckler & Koch later settled out of court, changing one product's name from "HK M4" to "HK416". However, on December 8, 2005, a District court judge in Maine granted a summary judgment in favor of Bushmaster Firearms, dismissing all of Colt's claims except for false advertising. On the latter claim, Colt could not recover monetary damages. The court also ruled that "M4" was now a generic name, and that Colt's trademark should be revoked.[59]

Trademark issues

Complicating the Army search for higher reliability in the M4 is a number of observations of M4 gas piston alternatives that suffer unintended design problems. The first is that many of the gas piston modifications for the M4 isolate the piston so that piston jams or related malfunction require the entire weapon be disassembled, such disassembly cannot be performed by the end user and requires a qualified armorer to perform out of field, whereas almost any malfunction with the direct-impingement system can be fixed by the end user in field. The second is that gas piston alternatives use an off-axis operation of the piston that can introduce carrier tilt, whereby the bolt carrier fails to enter the buffer tube at a straight angle, resulting in part wearing. The third is that the use of a sound suppressor results in hot gases entering the chamber, regardless of a direct-gas impingement or gas piston design choice. The gas-piston system also causes the firearm to become proprietary to the manufacturer, making modifications and changes with parts from other manufacturers difficult.[18][56] The argument for a gas piston is that it would reduce fouling; while the argument against it is that it would increase weight and reduce accuracy. The Enhanced M4 uses an articulating link piston operating system. An array of firearms accessory makers have offered gas piston conversion kits. The claimed benefits include superior reliability and performance and elimination of carrier tilt.

Gas piston

The reliability of the M4 has increased as the design was upgraded. In 1990, the M4 was required to fire 600 mean rounds between stoppages using M855 ammunition. In 2013, the current M4A1 version can fire 1,691 mean rounds between stoppages using M855A1 ammunition.[55]

In early 2010, two journalists from the New York Times spent three months with soldiers and Marines in Afghanistan. While there, they questioned around 100 infantrymen about the reliability of their M4 Carbines, as well as the M16 rifle. Troops did not report to be suffering reliability problems with their rifles. While only 100 troops were asked, they fought at least a dozen intense engagements in Helmand Province, where the ground is covered in fine powdered sand (called "moon dust" by troops) that can stick to firearms. Weapons were often dusty, wet, and covered in mud. Intense firefights lasted hours with several magazines being expended. Only one soldier reported a jam when his M16 was covered in mud after climbing out of a canal. The weapon was cleared and resumed firing with the next chambered round. Furthermore, a Marine Chief Warrant Officer reported that with his battalion's 700 M4s and 350 M16s, they've had no issues.[54]


[53] There were three extreme dust tests performed in 2007. In the Summer 2007 test, the M4 carbine stopped 882 times. The Fall 2007 results were very different from the other two tests; the M4 carbine had 148 class 1 stoppages due to rifle malfunctions and 148 class 1 stoppages due to magazine stoppages. The full-size M16 rifle had a total of 61 stoppages during the same extreme dust test.[52] The Army claimed that the M4 only suffered 296 stoppages, and that the high number reported could be attributed to discrepancies in the scoring process. If a certain number of malfunctions were found to be the result of a broken part, some of the stoppages counted could be eliminated in the final report. Colt also claimed that the testing conditions were unfair to the M4. Factors including the M4s used being taken from the Army inventory while the other rifles were provided directly from the manufacturers, and the carbine's burst fire operation when the others had fully automatic firing modes brought the validity of the results into question.[46][45] The Army began issuing an improved STANAG magazine in March 2009.[51]

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