M4 Carbine General Information

Jun 2nd 2020

M4 Carbine

From Wikipedia, the free encyclopedia

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.

The weapon is a shorter and lighter version of the M16A2 assault rifle, achieving 80% parts commonality with the M16A2. 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 M4A1 is sometimes also found with a heavier barrel to withstand heat from sustained fully-automatic fire.


Both the M4 and M4A1 carbines fire 5.56 x 45 mm NATO ammunition and are gas-operated, air-cooled, magazine-fed, selective fire firearms with a telescoping stock. A fixed stock can also be fitted, but this is not common practice in the U.S. military. The weapon is produced by Colt as the Model R0977, with the Colt Commando version as the Model R0933.

As with many carbines, the M4 is handy and more convenient to carry than a full-length rifle. While this makes it a candidate for non-infantry troops (vehicle crews, clerks and staff officers), it also makes it ideal for close quarters combat (CQC), and airborne and special operations. It has been adopted by United States Special Operations Command (USSOCOM) and is the preferred weapon of the U.S. Army Special Forces. Malaysia purchased M4 Carbine service rifles to replace the Steyr AUG service rifles in its armed forces in 2006.

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 M4, along with the M16A4, has mostly replaced M16 and M16A2 firearms; the U.S. Air Force, for example, plans to transition completely to the M4 Carbine. The M4 has also replaced the M3A1 submachine gun that remained in service (mostly with tank crews). The M4 is similar to much earlier compact M16 versions, such as the 1960s-era XM177 family, though unlike them it is not intended to fire the earlier M193/6 ball ammunition.

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 rail on top of the receiver for attachment of optical sights and other aiming devices — Trijicon TA11 and TA31 Advanced Combat Optical Gunsights (ACOG) and Aimpoint M68 Close Combat Optic (M68 CCO) being the favorite choices —, and a detachable rail-mounted carrying handle. The current government standards are the Colt Model 920 (M4) and 921 (M4A1).


The major difference between these models is that the M4 has a "S-1-3" (safe/semi-automatic/3-round burst) trigger group, while the M4A1 has a "S-1-F" (safe/semi-automatic/fully-automatic) trigger group.

M4 MWS (Modular Weapon System)

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 Accessory System (RAS) turns the weapon into the M4 MWS or Modular Weapon System.


The M4A1 carbine is a variant of the basic M4 carbine intended for special operations use. The M4A1 can be found in use by many U.S. military units, including the Delta Force, U.S. Navy SEALs, and the U.S. Marine Corps' Radio Reconnaissance Platoons and Force Reconnaissance companies. The M4A1 Carbine is specially 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 M4A1 does not have as far an effective range as the longer M16, many military analysts consider engagement with a non-specialized small arm above a range of 300 m to be unnecessary. It is extremely effective at 150 m or less.

In the last few years, M4A1 carbines have been being 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.


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 carbine, 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 (though many soldiers have changed the Trijicon reflex sight for M68 CCO red dot sights and EOTech holographic sights).


A second-generation SOPMOD kit (now known as SOPMOD II) is currently under development, with many different manufacturers competing for a contract. Notable manufacturers include Knight's Armament Company and their URX II, ARMS and their Selective Integrated Rail (SIR) system, along with Lewis Machine & Tool's Monolithic Rail Platform (MRP).

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 SFW built by Diemaco of Canada. Although Diemaco was purchased by Colt and renamed Colt Canada, the Diemaco names and related firearms were kept.

As mentioned, the M4 replaced the M3A1 "Grease Gun" submachine gun that remained in U.S. service, mainly with tank crews. They previously had M3s, but this was changed to two M4s and two M9 pistols ("personal defense weapons"). This was as much to increase capability as it was to change over from .45 ACP, as M3A1s could be configured to fire 9mm ammunition.


The M4/M4A1 5.56 mm 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 36 cm 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 Carbine has semi-automatic and three-round burst fire modes, while the M4A1 has "semi" and "full auto", with no three-round burst. The M4 Carbine achieves over 80% commonality with the M16A2 rifle and was intended to replace the M3 .45 ACP 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 cancelled.) The M4 Carbine is also capable of mounting the M203 grenade launcher.

Some features compared to a full-length M16A2 rifle include:

  • Compact size
  • Shorter barrel
  • Telescoping buttstock
  • Higher rate of fire
  • Selective fully-automatic rather than 3-round bursts (M4A1 only)

However, there have been some criticisms of the carbine, such as lower bullet 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 assault rifle, the M4 Carbine and the M4A1 Carbine can be fitted with many accessories, such as night vision devices, laser pointers, telescopic sights, bipods, the M203 grenade launcher and the XM26 LSS 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 Aimpoint. EOTech holographic weapon sights are to be part of the SOPMOD II package. Visible and IR (infrared) lights of various manufacturers are also commonly attached using various mounting methods.