m4 carbine

History and variants
For more details on M4 Carbine variants, see AR-15 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.

[edit] 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.

[edit] 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/PAQ-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 Accessory System (RAS) turns the weapon into the M4 MWS or Modular Weapon System.

[edit] M4A1
The M4A1 carbine is a fully-automatic variant of the basic M4 carbine intended for special operations use. The M4A1 is used by almost all U.S special operation units. The M4A1 Carbine is especially favored by counter-terrorist and special operations 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 to be unnecessary. It is effective at ranges of 150 meters or less and has a maximum effective range of about 500-600 meters.[6]
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.