M4 carbine

On 1 July 2009, the U.S. Army took complete ownership of the M4 design.[18] 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.[18] 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 Stoner expanding gas system with a gas piston system.

The benefits of these changes, however, have come under scrutiny from both the military and civilian firearms community.[19][20] According to a PDF detailing the M4 Carbine improvement plans released by PEO Soldier, the direct impingement system would be replaced only 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.[21]

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.[22]

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.[23] 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.[24] On 16 November 2012, Colt's protest of Remington receiving the M4A1 production contract was dismissed.[25] 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.[26][27] The order is expected to be completed by 2018.[28]

The M4 product improvement program (PIP) is the effort by the U.S. Army to modernize its inventory 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.[29]

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.[30] In June 2016, the M4A1+ was canceled after reviewing the offerings and determining that there were no major upgrades currently offered.[31]

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, flash suppressors, laser sights, 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, AN/PEQ-15 multi-mode laser, AN/PEQ-16 Mini Integrated Pointing Illumination Module MIPIM, TA31 or M150 Advanced Combat Optical Gunsight (ACOG), M68 CCO, grip pod etc. 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.

As for magazines, the M4 and the M4A1 can have 30-round box magazine or other STANAG magazines. Other types of magazines with different capacities such as the 100 rounds Beta C-Mag is also available.

In January 2017, a USMC unit deployed with suppressors mounted to every infantry M4 service weapon. Exercises showed that having all weapons suppressed improved squad communication and surprise during engagements; disadvantages included additional heat and weight, increased maintenance, and the greater cost of equipping so many troops with the attachment.[33] In July 2020, the Marine Corps announced it would be ordering suppressors for use by all M4 carbines used by close combat units.[34] The Marines began to rollout suppressors for all M4/M4A1 carbines in infantry, reconnaissance and special operations units in December 2020.[35]

M4 feedramps are extended from the barrel extension into the upper receiver. This can help alleviate feeding problems that 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 operating under its command. 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 TA01 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-inch length.

The XM177E2 was used in the military from 1967 until 1994, when it was replaced by the M4. The weapon, however, was at its prototype stage, and it was used as a testing weapon to see how it can be used in combat. In 1966, Colt introduced the XM177, which was an improved variant of the CAR-15 Carbine. The Colt 609 was used by the US Army, the US Navy Seals, and the MACV-SOG, referred as the XM177E1. The USAF however used the Colt 610, which is referred to as the GAU-5A, which became known as the GUU-5P. The XM177E1 does have problems with weapon jamming, however it does have positive reviews, and some soldiers preferred the XM177 over the M16. Mainly because it was shorter, lighter, and easier to handle. In 1967, Colt adopted the Colt 629, which is known as the XM177E2. The XM177E2 has some improvements over the M16A1, and Is prone to jamming less than the older variants. The weapon quickly became a favorite of the MACV-SOG. In 1982, Colt and the US Army start making a variant of the XM177E2 that fires 5.56mm NATO rounds called the XM4, and it was improve throughout the following years until 1991 when it became the M4.

M4 MWS (Modular Weapon System) shown with various accessories including M203 grenade launcher, RIS foregrip, removable carry handle/rear sight assembly, AN/PAQ-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.

M4A1 with TA01NSN ACOG 4×32 optical sight

The M4A1 carbine is a fully automatic variant of the basic M4 carbine intended for special operations use. The M4A1 was introduced in May 1991, and was in service in 1994. The M4A1 was the first M4 model with the removable carry handle. 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. It has a maximum effective range of 500 to 600 meters (550–660 yd).[4] The fully automatic trigger gives a more consistent trigger pull, which leads to better accuracy.[36] According to Mark A. Westrom, owner of ArmaLite, Inc., automatic fire is better for clearing rooms than burst fire.[37]

In the last few years, M4A1 carbines have been refitted or received straight from the 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.[38] 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. Three hundred conversions can be done per day to equip a brigade combat team per week, with all M4A1 conversions to be completed by 2019.[39][40]

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

The Mk 18 Close Quarters Battle Receiver is a variant of M4A1 with a 10.3-inch barrel upper receiver.[41] Current contractors for the Mk 18 are Colt and Lewis Machine & Tool (LMT) NSN 1005-01-527-2288.

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".[42] 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.[43] The Individual Carbine competition was canceled before a winning weapon was chosen.[36]

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 around 1995.

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 has been doubled (while retaining the weapon's 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.[44]

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.[47]

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, and 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 the 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.[48] Some issues have been addressed with the issuing of the improved STANAG magazine in March 2009,[49][50] and the M855A1 Enhanced Performance Round in June 2010.[51]

In the fall 2007, the Army tested the M4 against three other carbines in "sandstorm conditions" at Aberdeen Proving Ground, Maryland: the Heckler & Koch XM8, Fabrique Nationale de Herstal SOF Combat Assault Rifle (SCAR) and the Heckler & Koch HK416. Ten of each type of rifle were used to fire 6,000 rounds each, for a total of 60,000 rounds per rifle type.[52] The M4 suffered far more stoppages than its competitors: 882 stoppages, 19 requiring an armorer to fix. The XM8 had the fewest stoppages, 116 minor stoppages and 11 major ones, followed by the FN SCAR with 226 stoppages and the HK416 with 233.[53][54]

Despite 863 minor stoppages—termed "class one" stoppages, which require 10 seconds or less to clear, or "class two" stoppages, which require more than ten seconds to clear—the M4 functioned well, with over 98 percent of the 60,000 total rounds firing without a problem. The Army said it planned to improve the M4 with a new cold-hammer-forged barrel to give longer life and more reliable magazines to reduce the stoppages. Magazine failures caused 239 of the M4's 882 failures. Army officials said the new magazines could be combat-ready by spring if testing went well.[55] The Army began issuing an improved STANAG magazine in March 2009.[49][50]

According to the Army, the M4 only suffered 296 stoppages and said that the high number reported could be attributed to discrepancies in the scoring process. The Army testing command stated that, if the number of stoppages caused by a broken part met some threshold, they would be eliminated from the final report pending redesign of the part. Colt also claimed that the testing conditions were unfair to the M4, as the M4s used in the test were normal guns from active inventory, with remaining service life varying randomly. Further, the trial M4s had burst-mode fire groups, which are more complicated and prone to failure than the fully automatic fire groups the other manufacturers presented for testing.[56]

There were three extreme dust tests performed in 2007. The second Summer 2007 results showed a large difference from the later fall test with the M4 having 148 class 1 stoppages due to rifle malfunctions and 148 class 1 stoppages due to magazine stoppages. The full-size M16 rifle had 61 stoppages during the same extreme dust test.[57]

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 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 there were no issues with his battalion's 700 M4s and 350 M16s.[58]

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.[59]

During the 2009 Marine Corps Infantry Automatic Rifle testing, the Colt IAR displayed a MRBS of CLASS I/II Stoppages of 952 rounds, with a MRBEFF of Class III Stoppages of 60,000 rounds.[60]

An array of firearms accessory makers have offered gas piston conversion kits for the M4. The claimed benefits include less needed lubrication for the bolt carrier group to run reliably and reduced fouling. The argument against it is increased weight and reduced accuracy.[61] The Enhanced M4 uses an articulating link piston operating system.

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. This can also tilt the bolt during extraction, leading to increased bolt lug failures. 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 may also cause the firearm to become proprietary to the manufacturer, making modifications and changes with parts from other manufacturers difficult.[20][62]

Sales of select-fire or fully automatic M4s by Colt are restricted to military and law enforcement agencies. No private citizen can own an M4 in a select-fire or fully automatic configuration, as this model of fully automatic rifle was developed after the 1986 ban on fully automatic weapons available to be purchased by US citizens under the Firearms Owners' Protection Act of 1986. 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), who are 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 1986 would be legal to own by non-SOT civilians. The modular nature of the AR-15 design, however, makes it relatively simple to fit M4-specific components to a "pre-'86" select-fire AR-15 lower receiver, producing an "M4" in all but name.

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

• Colombian conflict (1964–present)
• Civil conflict in the Philippines (1969–present)
• Kosovo War (1998–1999), first US military usage of the M4 carbine.

• War in Afghanistan (2001–present)
• Iraq War (2003–2011)
• Lebanon War (2006)
• Mexican Drug War (2006–present)
• Russo-Georgian War (2008)[128]
• Gaza War (2008–2009)

• Syrian Civil War (2011–present)[129]
• Lahad Datu standoff (2013)[130]
• Iraqi Civil War (2014–2017)[129]
• Battle of Arsal (2014)
• Battle of Marawi (2017)[131]

• XM8 rifle: canceled in 2005
• Close Quarters Battle Receiver: successful replacement
• Individual Carbine: canceled in 2013. Candidates included:
  • Barrett REC7 PDW
  • Beretta ARX 160
  • Remington ACR
  • FN SCAR
  • Heckler & Koch HK416
  • LWRC M6A4
  • Robinson Arms XCR
  • SIG 556
  • Colt ACC-M