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Carbine, 5.56 mm, M4
A M4A1 with SOPMOD package, including Rail Interface System (RIS) and Trijicon ACOG 4x.
An M4A1 with SOPMOD package, including Rail Interface System (RIS), flip-up rear sight and Trijicon ACOG 4x.
Type Assault rifle
Place of origin  United States
Service history
In service 1997–present
Used by See Users
Wars War in Afghanistan (2001–present), War in Iraq (2003-present), Colombian Civil War
Production history
Manufacturer Colt Defense
Produced 1994–present
Variants M4A1, CQBR
Weight 5.9 lb (2.7 kg) empty
6.9 lb (3.1 kg) with 30 rounds
Length 33 in (838 mm) (stock extended)
29.8 in (757 mm) (stock retracted)
Barrel length 14.5 in (368 mm)

Cartridge 5.56x45mm NATO
Action Gas-operated, rotating bolt
Rate of fire 700-950 round/min cyclic
Muzzle velocity 2,970 ft/s (910 m/s)
Effective range 500 m (550 yd)
Feed system 30 round box magazine or other STANAG Magazines.

The M4 carbine is a family of firearms tracing its lineage back to earlier carbine versions of the M16, all based on the original AR-15 made by ArmaLite. It is a shorter and lighter version of the M16A2 assault rifle, with 80% parts commonality with the M16A2.[1] The M4 has selective fire options including semi-automatic and three-round burst (like the M16A2), while the M4A1 has a "full auto" option in place of the three-round burst.



The M4 and variants fire 5.56x45mm NATO ammunition and are gas-operated, air-cooled, magazine-fed, selective fire firearms with a 4-position telescoping stock. 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.[2] 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, however most of the similarities are not very noticeable.

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 nearly 6" (15 cm) longer barrel. This becomes most apparent at ranges of 300 yards and beyond. Statistically, however, most small-arms engagements occur within 100 yards. This means that the M4 is very much an adequate weapon for the majority of troops. The marginal sacrifice in terminal ballistics and range, in exchange for greatly improved handling characteristics, is usually thought to be a worthwhile compromise.[citation needed]

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 was developed and produced for the United States government by Colt Firearms, which has an exclusive contract to produce the M4 family of weapons through 2009; however, a number of other manufacturers offer M4-like firearms. The M4A1, along with the M16A4, have mostly replaced the M16A2; the U.S. Air Force, for example, plans to transition completely to the M4. The M4 is also the standard rifle for U.S. Air Force Security Forces members whether at home station or deployed abroad. They maintain a yearly qualification on it.

The United States Marine Corps has ordered its officers (up to the rank of lieutenant colonel) and Staff Non-commissioned officers to carry the M4A1 carbine instead of the M9 handgun. This is in keeping with the Marine Corps motto, "Every Marine a rifleman." United States Navy corpsmen will also be issued M4A1s instead of the M9.[3]

History and variants

Except for the very first delivery order, all U.S. military-issue M4 and M4A1 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 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. The M4A1 is especially favored by counter-terrorist and special forces units for close quarters combat because of the carbine's compactness and firepower. These features are also very useful in urban warfare. Although the M4 has less effective range than the longer M16, many military analysts consider engagement with a non-specialized small arm above a range of 300 meters (330 yd) to be unnecessary. It is effective at ranges of 150 meters (160 yd) or less and has a maximum effective range of about 500 to 600 meters (550–660 yd).[4]

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, which is useful due to the complaints of high-heat production from test soldiers, which occurs 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.


SOPMOD (Special Operations Peculiar Modification) Block I

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 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) is currently under development, with many different manufacturers competing for the contract. Notable bidders include Knight's Armament Company, Atlantic Research Marketing Systems (ARMS), and Lewis Machine & Tools. Daniel Defense has won the contract for the RIS-II, the next generation of rail handguards.

Future replacement

On November 13, 2008, the U.S. Army hosted an invitation-only Industry Day regarding a potential future replacement for the M4 carbine. Nineteen companies provided displays and briefings for military officials. The weapons displayed included the Barrett REC7 PDW, Bushmaster ACR, FN SCAR, Heckler & Koch HK416, Heckler & Koch XM8, LWRC M6A4, Robinson Arms XCR, SIG 556, as well as Colt's own improved version of the M4, the Colt ACC-M (Advanced Colt Carbine-Monolithic). The goal of the Industry Day was to provide officials with knowledge as to the current state of the art, which will assist the writing of a formal requirements document.[5]

On July 1, 2009, the U.S. Army took complete ownership of the M4 design.[6] This will allow companies besides Colt to compete with their own M4 designs. The Army is planning on fielding the last of its M4 requirement in 2010.[6]

On October 30, 2009, Army weapons officials proposed a series of changes to the M4 to Congress. Requested changes include an electronic round counter that records the number of shots fired, a heavier barrel, and replacing the direct impingement system with a gas piston system.[7]


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

The M4/M4A1 5.56mm carbine is a gas-operated, air-cooled, magazine-fed, selective fire, shoulder-fired weapon with a telescoping stock. A shortened variant of the M16A2 rifle with a 14.5 in (368 mm) barrel, the M4 provides the individual soldier operating in close quarters the capability to engage targets at extended range with accurate, lethal fire. The original M4 has semi-automatic and three-round burst fire modes, while the M4A1 has "semi" and "full auto", with no three-round burst. The M4 has over 80% commonality with the M16A2 rifle and was intended to replace the .45 ACP M3 submachine guns and selected M9 pistols and M16 rifle series with most Army units (this plan was thought to be changed with the development of the XM29 OICW and the XM8 carbine. However, both projects were canceled). The M4 is also capable of mounting an M203 grenade launcher, the M203A1 with a 9-inch barrel as opposed to the standard 12-inch barrel of the M203 used on the M16 series of rifle.

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

  • Compact size
  • Shortened barrel 14.5 in (368 mm)
  • 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 M4 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, 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).


An April 2002 presentation by the US Army Natick Soldier Center presented by LTC Charlie Dean and SFC Sam Newland reported on lessons learned from M4 use in Afghanistan (such as use during Operation Anaconda):[citation needed]

  • 90% of soldiers reported confidence in the weapon.
  • 20% were dissatisfied with its ease of maintenance.
  • 34% of soldiers reported that their M4's handguards rattled and became excessively hot when firing.
  • 15% reported that they had trouble zeroing the M68 reflex sight.
  • 35% added barber brushes and 24% added dental picks to their cleaning kits.
  • Soldiers reported the following malfunctions:
    • 20% reported double-feeding.
    • 15% reported feeding jams.
    • 13% reported that feeding problems were usually due to magazines.

Soldiers requested the following changes:

  • 55% requested the firearm be made lighter.
  • 20% requested a slightly larger magazine.

2007 dust test

In the fall of 2007, the Army tested the M4 against three other carbines in "sandstorm conditions" at Aberdeen Proving Ground, Maryland: the Heckler & Koch XM8 rifle, 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.[8] 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.[9][10] The Army was quick to point out that even with 863 minor stoppages—termed "class one" stoppages which require 10 seconds or less to clear and "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.[11]

Trademark issues

A U.S. Marshal covers a doorway with an M4.

Colt previously held a U.S. trademark on the term "M4".[12] Many manufacturers have production firearms that are essentially identical to a military M4. Civilian models are sometimes colloquially referred to as "M4gery"[13] (pronounced ĕm'fôr jə-rē, 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.[14]


U.S. citizen ownership

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. However, as US firearms law considers the lower receiver of a M16/M4 type rifle to be the "firearm" (the serial numbered and, in the case of machine guns, registered under federal law, part of the weapon).[citation needed] Therefore the more common registered Colt M16 may be configured as an M4 by replacing the M16 upper receiver/barrel assembly with an M4 upper half, and replacing the fixed rifle stock with a 4 or 6-position telescoping M4 stock.


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  2. ^ "Photo of the Colt M4 with the redesigned telescoping stock". Colt Defense. Retrieved 2008-09-13. 
  3. ^ "New Assignment Rationale for Individual Weapons". U.S. Marine Corps, June 22, 2007.
  4. ^ "M-4 Carbine". U.S. Army Fact Files. United States Army. Retrieved 2008-09-13. 
  5. ^ Matthew Cox (2008-11-25). "Army considers options in replacing the M4". Army Times. Retrieved 2009-03-25. 
  6. ^ a b Matthew Cox (2009-07-07). "Army acquires rights to M4". Army Times. Retrieved 2009-08-18. 
  7. ^ Matthew Cox (2009-11-22). "Major revamp possible for M4 carbine". Army Times. Retrieved 2009-11-26. 
  8. ^ Lowe, Christian (2007-12-18). "M4 Carbine Fares Poorly in Dust Test". Military Advantage.,13319,158468,00.html. Retrieved 2008-09-13. 
  9. ^ "...And Here's the Rest of the M4 Story". Defense Tech. Military Advantage. 2007-12-18. Retrieved 2008-09-13. 
  10. ^ Cox, Matthew (2007-12-19). "Newer carbines outperform M4 in dust test". Army Times. Army Times Publishing Company. Retrieved 2008-09-13. 
  11. ^ Cox, Matthew (2007-12-17). "M4 may get tougher barrel, better mags". Army Times. Army Times Publishing Company. Retrieved 2008-09-13. 
  12. ^ US Trademark serial number 76335060 registration number 2734001
  13. ^ "m4gery". Urban Dictionary. Retrieved 2008-09-13. 
  14. ^ OpenJurist synopsis of denial of Colt's appeal to 08 Dec 2005 ruling
  15. ^ Petty Officer First Class David Votroubek (July-August 2008). "New Gear for Afghan Commandos". United States Army Logistics Management College. Retrieved 2009-03-24. 
  16. ^ Soraya Sarhaddi Nelson (2007-07-23). "New Afghan Commandos Take to the Frontlines". National Public Radio. Retrieved 2009-03-24. 
  17. ^ a b c d Daniel Watters. "The 5.56 X 45mm: 2006". Retrieved 2009-03-25. 
  18. ^ a b c d e f g h i j k l m Daniel Watters. "The 5.56 X 45mm: 2008". The Gun Zone. Retrieved 2009-01-27. 
  19. ^ Victor Epand. "Bull Pup Guns Configuration". Retrieved 2009-03-24. 
  20. ^ "M4 Carbine". Asia Pacific Defence Solutions Group. Retrieved 2009-01-20. 
  21. ^ a b Hogg, Ian (2002). Jane's Guns Recognition Guide. Jane's Information Group. ISBN 0-00-712760-X.
  22. ^ a b c Daniel Watters. "The 5.56 X 45mm: 2007". Retrieved 2009-03-25. 
  23. ^ Daniel Watters. "The 5.56 X 45mm: 2008". Retrieved 2009-09-11. 
  24. ^ "Greece Ministry of Public Order Press Office: Special Anti-Terrorist Unit". Official Website of the Hellenic Police. July 2004. Retrieved 2009-10-13. 
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  26. ^ Joseph Giordono (2007-05-16). "Iraqi soldiers switching over to M-16s and M-4s". Stars & Stripes. Retrieved 2009-03-25. 
  27. ^ CJSOTF-AP Public Affairs. "U.S. Special Forces, Iraqi army ops: Raids result in 102 detainees, large weapons cache, no losses". Retrieved 2009-03-25. 
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  29. ^ "全文掲載:飯柴大尉の声明文" (in Japanese). Retrieved 2009-01-12. 
  30. ^ Christopher J. Castelli (September 2008). "Department of Defense to equip Lebanon's Special Forces with Small Arms, Vehicles". DISAM Journal. Retrieved 2009-02-08. 
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  32. ^ "M4 carbine also used by Royal Malaysian Police, Malaysian Maritime Enforcement Agency and others". SME Ordnance Sdn Bhd. Retrieved 2008-09-13. 
  33. ^ Thompson, Leroy (December 2008). "Malaysian Special Forces". Special Weapons. Retrieved 2009-12-17. 
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  36. ^ Daniel Watters. "The 5.56 X 45mm: 1990-1994". Retrieved 2009-03-25. 

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