M16A4

The M16A4 is a selective-fire assault rifle capable of semi-automatic and three-round burst firing modes, and is an updated version of the M16A2 rifle. The M16A4 rifle differs from the M16A2 in that it has a MIL-STD-1913 Picatinny rail under the removable carrying handle which allows different types of optics to be mounted on the top of the receiver. There are also Picatinny rails mounted on the handguard to allow lasers, flashlights, foregrips, and grenade launchers. It is the current standard issue rifle of the US Marine Corps and some US Army units. The US military purchases M16A4s from Colt and FN Manufacturing with most M16s in recent orders being made by FN. In addition, M16 receivers for sniper variants and special purpose weapons have been procured from ArmaLite and Knight's Armament Company. The M16A4 is light and accurate, but requires frequent cleaning, partially because it is susceptible to dirt, but also because its internal mechanism lacks a piston; the gases strike the bolt of the rifle directly. This method of operation results in a lighter and simpler rifle, but causes reliability issues, as carbon is blown into the chamber of the weapon.

History
The M16 was originally designed in 1957 by Eugene Stoner as the AR-15, a 5.56x45mm variant of his earlier, and unpopular, AR-10 in 7.62x51mm. It was accepted by the U.S. Army in 1961 and saw use in Vietnam, where it was generally unpopular due to fouling problems caused by the weapon's direct gas impingement system and the Army using a new type of propellant the weapon wasn't originally designed to use. This led to the design and adoption of the M16A1, which featured a forward assist. This feature, combined with regular maintenance of the weapon, helped alleviate jams and other problems caused by fouling.

The M16A2 rifle was adopted in the 1980s. The M16A2 was followed by the M16A3, which reverted to the full-automatic option. The M16A3 saw little use in infantry, but was used in some special purposes by the Navy.

The M16A4 and its variant, the M4 Carbine, are in wide use by United States military forces, particularly Marine Corps. The Army now primarily issues the M4, which has not seen widespread use in the Marines.

Call of Duty 4
In singleplayer, the M16A4 can only be picked up during War Pig. It is dropped occasionally (sometimes with the M203) by dead Marines, and always uses iron sights. The M16A4 in singleplayer has a fully automatic firing mode, which is inaccurate, as in reality the M16A4 does not feature full-automatic fire. It is used by Lt. Vasquez.

In multiplayer, the M16A4 is available at level 1 and fires in three-round bursts. It can be fitted with a silencer, Red Dot Sight, ACOG Scope, and M203. It is also fairly similar to the M4 Carbine, as it has the same appearance, the same iron sights, and they also share ammo with each other, as well as with the G36C, however, the M16 has noticably higher damage than the M4.

Using
It is often recommended to fire at a distance, as the M16A4's three-round burst is not very effective in close quarters combat. Steady Aim does a good job at making up for this, but should not be heavily relied upon. When used with Stopping Power, the M16 is able to kill in one burst at any range provided that all three shots hit the target (only two are needed at medium and close range) and the target is not behind a wall or using Juggernaut.

Double Tap decreases the delay between the firing of each of the three bullets in a single burst and decreases the amount of time between bursts. With Double Tap, all of the bullets fired in a single burst are now fired at almost the exact same time. This can be somewhat useful in many situations, as there is a higher chance of all three bullets hitting the target. Stopping Power is still considerably better, except in Hardcore modes.

The M16A4 is widely considered to be one of the best guns in the game due to its power, accuracy, reload, and speed. This makes it popular to use, but is also frowned upon by some players because it is easy to kill people with because all an M16A4 user has to do is fire one to two times, and the bursts are so fast. It is also commonly used in gamebattles for these reasons.

Call of Duty 4 DS
The DS variant of the M16A4 is quite different compared to the one in the console games. It fires in automatic mode (possibly similar to the M4A1), but it has a slow rate of fire. However, it does more damage per shot to compensate for this.

Call of Duty: Modern Warfare 2
The weapon is set to return in Call of Duty: Modern Warfare 2. It's stats are currently unknown.

Trivia

 * The words "BLUE4" and "Property of U.S. Government, Caliber 5.55x45mm NATO, M4A1 Carbine" in bold blue letters is visible on the side of the M16A4. The M16 fires 5.56x45mm NATO cartridges (5.55 is likely a typo) and the M4 Carbine was derived from the M16 series. Infinity Ward most likely used the same weapon skin for both the M4 and the M16A4.
 * Individual bursts fire as fast as the P90 and M249 SAW, making it an unusually fast-firing Assault Rifle.
 * In multiplayer, if you attach the grenade launcher to the M16A4, the heat shield turns into the one that was used for mounting the M203 onto M16A2s. This is unnecessary, as the M5 RAS used on the M16A4 is just as capable of mounting the M203. This does not necessarily indicate that it turns into an M16A2, since it is one part that is very easy to swap out, and it still has the A4's removable carrying handle.
 * In singleplayer, if you enable the "Infinite Ammo" cheat and pick up a M16A4 with the M203, the full-automatic rate of fire of the grenade launcher is very, very fast and greatly surpasses that of the M4A1. Curiously, the M203 on the M4 Carbine and the GP-25 on the AK-47 have much slower full-automatic rates of fire, about the same as the W1200.
 * The M16A4 and the FAMAS are the only guns in the Call of Duty series that is capable of firing in bursts.
 * In-game model depicted the M16A4 having A4 upper receiver,but has an A1 lower receiver,which explains why the weapon can go full-auto in Single player.
 * In Call of Duty 4 (DS), the M16 has an EOTech sight in single player, instead of an iron sight in multi-player.
 * When an M203 is attached the forward iron sight changes size.