M4A1 Carbine

The M4A1 Carbine is a short-barreled, telescoping-stocked variant of the M16A2, derived from the AR-15. The M4A1 Carbine fires the same 5.56x45mm round as the M16, but the M4A1 version is capable of firing on full automatic as opposed to the M16A2, M16A4 and M4's three round burst. The M4 Carbine was designed to be a defensive weapon for personnel whose role may require a powerful defensive weapon when a full-sized rifle may be impractical. The M4 is also used by various special forces for its compact nature and full-power rifle cartridge, making it ideal for close quarters combat. To this end, a variety of modifications for the M4A1 Carbine, known as the Special Operations Peculiar MODification (SOPMOD) kit, are issued to US military special operations forces, which include the Knight's Armament Company vertical foregrip, M203, Knight's Armament Company QDSS-NT4 Suppressor, AN/PEQ-2A, ACOG Scope, and other accessories. The US Army's units vary between keeping the M16, issuing the M16 to some soldiers and the M4 to others, and replacing the M16 entirely, and the US Marine Corps still largely uses the M16. The M4 is also used by many other countries including Israel, Japan, Australia, Greece and Mexico.

Call of Duty 4
In singleplayer, the M4A1 (with a Tasco Red Dot Sight, AN/PEQ-2A, and foregrip) is a frequent Marine drop and infrequent SAS drop. It is the starting weapon in Charlie Don't Surf and Game Over and one of the optional weapons at the beginning of Hunted. The "M4A1 Grenadier", which uses an EOTech Holographic Weapon Sight, AN/PEQ-2A and M203 is the starting weapon in The Bog, War Pig and Shock and Awe. The "M4A1 SOPMOD" uses the red dot sight, M203 grenade launcher, AN/PEQ-2A, and silencer and is the starting weapon in the majority of MacTavish's missions. Captain Price uses it in all missions. Surprisingly, in the SAS missions the M4A1 SOPMOD seems to have all attachments available, a red dot sight, silencer, grip (though the M4A1 Carbine has a grip, this does not serve to decrease recoil and does not count as an attachment. Note that the grip does not show from the third-person perspective in-game, as when dropped or held by another character, in both single-player and multiplayer), and an M203.

In multiplayer, the M4A1 Carbine has an attached foregrip. It can be modified with the Red Dot Sight, ACOG Scope, M203, or silencer. The M4A1 Carbine is fully automatic with moderate damage and very low recoil, but it has very high idle sway (4 degrees) and idles very fast, making it more difficult to aim with at long range. This was done for balancing purposes because the M4A1 was incredibly overpowered in the beta, in which it had no idle sway. The M4A1 also shares ammo with the M16A4 and the G36C. The M4A1 Carbine is one of the most commonly used weapons in online multiplayer on all platforms.

Weapon Performance
As stated above, the M4A1 is not particularly effective at long range due to its sway on consoles. In the PC version, however, idle sway is easily compensated for with the mouse, making it very effective at all ranges. The M4 is ideal in close quarters combat due to its high rate of fire. Steady Aim makes the M4 a fairly effective hip-fire weapon. Double Tap can be quite effective due to low recoil and low damage, but ammo use increases dramatically.

The weapon's low damage output can be compensated by its high rate of fire, and the very low recoil allows for accurate full automatic fire at short and medium ranges. Because of this, Bandolier can be useful.

Modern Warfare 2
The M4A1 has retained most qualities and appearances of the Call of Duty 4 version. The M16-type carry handle has been replaced with foldable sights, and attachments available are the Red Dot Sight, ACOG Scope, M203, silencer, under-barrel Shotgun, and Heartbeat Sensor along with others that have yet to be released.

Trivia

 * On the lower receiver of the M4A1, it says " BLUE4 ". It also says, "Property of U.S. Government, Caliber 5.55mm, M4A1 Carbine" over the magazine housing. However, the M4A1 Carbine fires a 5.56mm, not a 5.55mm, and it is also used by the SAS in game.
 * The SAS has never actually used the M4, but rather the cosmetically similar Diemaco C8SFW.
 * A newly released concept art screenshot of Call of Duty: Modern Warfare 2 shows Soap holding a Bushmaster M4, a licensed copy of the original Colt M4. On the magwell is the Bushmaster logo. It uses a different set of sights than the M16-type sights used on the Colt version.
 * If you attach an M203, the recoil will be slightly increased, but not by much. This also applies with the M16A4. This is probably because when you attach the M203, you remove the grip.
 * If you set the sensitivity on console versions to 10 (max) then spin in circles clockwise (to the right) the right side of the gun can be slightly visible, showing a much less detailed texture. This was done because that side of the gun is not normally visible and not detailing it saves memory.
 * The M4 Carbine's reload animation is used for the ACR in Modern Warfare 2.
 * The M4 and the M16A4's front sights disappear when another optic is attached.
 * It is the signature weapon of Price.