|Carbine, Caliber 5.56 mm, M4|
|Place of origin||United States|
|Used by||See Users below|
|Wars||Colombian conflict|
Burundian Civil War
War in Afghanistan (2001–present)
Syrian Civil War
2013 Lahad Datu standoff
Iraqi Civil War (2014–2017)
Yemeni Civil War (2015-present)
Battle of Marawi
|Manufacturer||See Manufacturers below|
|Unit cost||$700 (avg. cost)|
Mark 18 Mod 0 CQBR
|Weight||6.5 lb (2.9 kg) empty |
7.49 lb (3.40 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)|
|Caliber||5.56 mm (.223 in)|
|Action||Gas-operated, rotating bolt (Direct impingement)|
|Rate of fire||700–950 round/min cyclic|
|Muzzle velocity||2,970 ft/s (910 m/s) (M855A1 round)|
|Effective firing range||500 m (550 yd)|
|Feed system||30-round box magazine or other STANAG magazines.|
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 extensively used by the United States Armed Forces and is largely replacing the M16 rifle in United States Army and United States Marine Corps combat units as the primary infantry weapon and service rifle.
The M4 is also capable of mounting the M203 and M320 grenade launchers. The distinctive step in its barrel is for mounting the M203 with the standard hardware. The M4 has semi-automatic and three-round burst firing modes (like the M16A2 and M16A4), while the M4A1 has semi-automatic and fully automatic firing modes (like the M16A1 and M16A3).
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 leading "to considerable muzzle flash and blast, so that a large flash suppressor had to be fitted". "Nevertheless, as a short-range weapon it is quite adequate and thus, [despite] its caliber, [the XM177 'Commando'] is classed as a submachine gun." In 1984, Colt began work on a new carbine design called the XM4 combining the best features of the Colt Commando and later the M16A2 rifles.
In 1984, the first model was made, and it was tested in May 1985. The first models had an upper receiver with an A1 sight, and were given a shorter 11.5-inch barrel, but later ones were given a longer 14.5-inch barrel for the bayonet and the M203 Grenade Launcher. The second model was made in May 1986, and it was tested from May 1986 though May 1987; at the time it had an A2 Upper Sight, and it had 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 cartridge deflector, as well as other minor refinements. In May 1991, the XM4 was renamed to the M4, and Colt made a manual.
The M4 was officially accepted into service by the U.S. military in 1994, and first saw action in the hands of U.S. troops deployed to Kosovo in 1999 in support of the NATO-led KFOR peacekeeping force. It would subsequently be used heavily by U.S. forces during the Global War on Terrorism, including Operation Enduring Freedom and Operation Iraqi Freedom. In the U.S. Army, the M4 had largely replaced M16A2s as the primary weapon of forward deployed personnel by 2005. The M4 carbine also replaced most submachine guns and selected handguns in U.S. military service, 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. 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. While ordinary riflemen in the Marine Corps were armed with M16A4s, M4s were 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 had 80,000 M4 carbines in their inventory.
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 ability to address modern combat situations that happen mostly 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 had the 17,000 M4s in the inventory needed to outfit all infantrymen who needed one. 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 was to begin in early 2016 and be completed by September 2016. In December 2017, the Marine Corps revealed a decision to equip every Marine in an infantry squad with the M27, replacing the M4 in that part of the service. MARSOC will retain the M4, as its shorter barrel is more suited to how they operate in confined spaces.
On 1 July 2009, the U.S. Army took complete ownership of the M4 design. 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. 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. 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.
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.
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. 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. On 16 November 2012, Colt's protest of Remington receiving the M4A1 production contract was dismissed. 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. The order is expected to be completed by 2018.
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.
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. In June 2016, the M4A1+ was canceled after reviewing the offerings and determining that there were no major upgrades currently offered.
The M4 and its variants fire 5.56×45mm NATO (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.
The M4 is a shorter and lighter variant of the M16A2 rifle, with 80% parts commonality. The M4 is similar to much earlier compact M16 versions, such as the 1960s-era XM177 family. Some of those visual similarities are obvious in both weapons.
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 (180 m) 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 vehicle crews.
Some features of the M4 and M4A1 compared to a full-length M16-series rifle include:
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.
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, AN/PEQ-15 multi-mode laser, AN/PEQ-16 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.
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.
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.
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.
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 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.
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 was introduced in May 1991, and was in service in 1994. 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 about 500 to 600 meters (550–660 yd). The fully automatic trigger gives a more consistent trigger pull, which leads to better accuracy. According to Mark A. Westrom, owner of ArmaLite, Inc., automatic fire is better for clearing rooms than burst fire.
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. 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.
The Mk 18 Close Quarters Battle Receiver is an M4A1 with a 10.3-inch barrel upper receiver. Current contractors for the Mark 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". 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. The Individual Carbine competition was canceled before a winning weapon was chosen.
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. The Colt Model 733, is their first design, and it was made in 1987. It was referred to as the M16A2 Commando, and later the M4 Commando. Unlike the XM177, the Colt Commando was a shorter variant of the M16A2. 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 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.
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 weapon. 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. Despite this, in post-combat surveys, 94 percent of soldiers rated the M4 as an effective weapons system.
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.
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. Some issues have been addressed with the issuing of the improved STANAG magazine in March 2009, and the M855A1 Enhanced Performance Round in June 2010.
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. 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.
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. The Army began issuing an improved STANAG magazine in March 2009.
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.
There were three extreme dust tests performed in 2007. The 2nd 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.
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 there were no issues with his battalion's 700 M4s and 350 M16s.
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.
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.
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. 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.
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The M4 was developed and produced for the United States government by Colt Firearms, which had an exclusive contract to produce the M4 family of weapons through 2011. However, a number of other manufacturers offer M4-like firearms. Colt previously held a U.S. trademark on the term "M4". Many manufacturers have production firearms that are essentially identical to a military M4, but with a 16" barrel. The Bushmaster M4 Type Carbine is a popular example. Civilian models are sometimes colloquially referred to as "M4gery" (// em-FOR-jər-ee, a portmanteau of "M4" and "forgery"). Colt had maintained that it retains sole rights to the M4 name and design. Other manufacturers had long maintained that Colt had been overstating its rights, and that "M4" had now become a generic term for a shortened AR-15. In April 2004, Colt filed a lawsuit against 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.
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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 full 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 full 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.
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