M16 Assault Rifle

The M16A3 Assault Rifle is a Tier 2 replacement for the submachine gun with higher damage and better accuracy. It holds 50 rounds with a 360 round reserve, having less total ammo than the Uzi, but being a better overall weapon. The M16 is the best choice as a general use firearm, as it is adaptable to most situations.

Tactics
The assault rifle is a weapon that is mostly effective in medium-range combat, as opposed to the short-range auto shotgun and the long-range hunting rifle. It is a very flexible weapon, however, adept at picking off Special Infected from a distance and piercing multiple Common Infected. It has quite a high piercing power, but not quite as high as the hunting rifle. Its high piercing power makes it is a useful weapon for clearing rooms full of Infected or destroying oncoming hordes

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With Valve's recent update reducing auto shotgun damage against a Tank and the assault rifle's high medium-range accuracy, the assault rifle has overtaken the auto shotgun as a weapon of choice for killing a Tank. The auto shotgun is still just as useful against nearby Infected, and in close quarters weapon choice lies to player preference.

Assault rifle ammo is easy to waste. The weapon trades ammo capacity for accuracy and damage, and this must be used to full advantage to be effective. The assault rifle generally needs only two or three shots to do its job, and any more hampers its accuracy at mid to long range. Thus, burst firing or "headhunting" is highly recommended at medium to long ranges as this not only helps to improve accuracy, but this also has the benefit of conserving precious ammo. A technique that seems to work pretty well on Xbox 360 to conserve ammo is to do a quick full trigger squeeze, which will fire 2 rounds. This can be useful when shooting long range and is effective on most difficulty settings except for expert.

While this weapon may be useful on higher difficulties because of its tighter cone of fire being safer when firing near other Survivors, this weapon struggles to keep up with the increased health of the Common Infected, requiring 4 bullets to kill one on expert, whereas a single shot from the other two weapons is still a kill. Additionally, the mid-range effectiveness of the assault rifle is shared with the pistols, meaning that taking the assault rifle sacrifices some flexibility.

In a group of four on finales, it's a good idea to have two people with auto shotguns and two people with assault rifles, get in a closet or similar point which causes the Infected to file in slowly, the two people with the shotguns crouch and shoot or use melee, while the two people with the assault rifles stand behind them and fire rounds into the Infected as they approach. Be careful to note when the Tank comes, however—a Tank can easily destroy a group of four Survivors hiding in the same closet. It's best to run out and kill it in open ground, and return to the former spot when the horde has come back.

Left 4 Dead 2
It appears there will be two new assault rifles in Left 4 Dead 2, a SCAR and an AK-47. The SCAR has a 60-round magazine and fires in three-round bursts, but the AK has a 40-round magazine and a slightly-longer reload time. The M16 will appear in Left 4 Dead 2 as well.

Behind the Scenes
Early screenshots show that the M16 lacked text on it's left side at one time.

When the M16 was drawn in early versions of the game, the character would pull the charging handle as soon as he took the gun out. The sound for this animation is still played when the M16 is drawn out in the final version of the game.

The Assault Rifle is based on Counter-Strike ' s M4A1 assault rifle. Its "price" in the gun's weapon file is the same as the M4A1's, and it references the M4A1 in its "animation" listing. Its firing sounds are also from the M4A1, but are slightly modified.

Trivia

 * The in-game texture states that this rifle is produced by the fictional company American Armory as the M16A2. However, it is actually based on the M16A3. The M16A3 rifle is an M16 class rifle with fully automatic fire mode replacing the 3 round burst used on the M16A2. In Left 4 Dead 2, it appears that the M16 is going to be changed to fix this error, as like the Combat Rifle, it will now fire in 3 round bursts. It is unknown whether a fully automatic M16A3 varient will be available.
 * It has 360 reserve ammo, so it may have originally had a capacity of 30 rounds, or could be an in-joke aimed towards X-Box 360 gamers who also play the game.
 * The M16 was the standard American weapon during the Vietnam War, of which Bill is a veteran. This could explain why he chooses this gun whenever given the opportunity, and also why he will sometimes remark, "Damn, now this takes me back!" upon doing so.