Colt M-4

The M4 Carbine is a family of firearms tracing its lineage back to earlier carbine versions of the M16,such as the XM177E2, all based on the original AR-15 made by ArmaLite.

The weapon is a shorter and lighter version of the M16A2assault 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. Each unit cost $1500 USD.

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 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).

M4/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/R.I.S)

M4 MWS (Modular Weapon System) shown with various accessories including M203 grenade launcher, RIS foregrip, removeable carry handle/rear sight assembly, AN/PEQ-2 laser system, and several optional 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 Accessory System (RAS) turns the weapon into the M4 MWS or Modular Weapon System.

M4A1

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, U.S. Army Rangers, 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 M4 does not have as great an effective range as the longer M16, many military analysts consider engagement with a non-specialized small arm above a range of 300 meters to be unnecessary. It is effective at ranges of 150 meters or less. It has a maximum effective range of about 400 meters.

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.

SOPMOD Block IEdit

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).

SOPMOD Block II

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,Daniel Defense RIS II Omega Rail , 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 9 mm ammunition.

MK 18 MOD 0

The MK 18 MOD 0 is a variant that is a CQB version with a 10.5 inch barrel.Equipped with a Daniel Defense M4 Omega Rail Systems. It is used primarily by US Navy SEALs. It was designed by the Naval Surface Warfare Crane Division.