The Tier 2 Automatic Rifles of Left 4 Dead 2 have a few alternative names given to them by the community or Valve. To assure that everyone knows which gun is which, I will list a few of the names I have heard.
The M-16 Assault Rifle was simply referred to as the Assault Rifle in the first Left 4 Dead, but as more guns were included in the sequel it came to be known by the name of the weapon it is based on, the M-16. It is occasionally confused for an M4A1 Carbine, a visually similar weapon that is available in Counter-Strike, another one of Valve's games.
The Combat Rifle is also referred to as a Desert Rifle in certain game files. It may be known as a Burst Rifle since it has mandatory burst fire in-game. It is based on the SCAR-L and SCAR-H.
The AK-47 is more or less a household name as far as Automatic Rifles go. It may simply be referred to as "AK" or "47".

Every Automatic Rifle has a unique rate of fire and damage dealt, as well as overall accuracy.

The M-16 Assault Rifle takes 2.35 seconds to be reloaded, the AK-47 needs 2.5 seconds, and the Combat Rifle requires 3.35 seconds. Reloading during a Horde is simple for the M-16 Assault Rifle or AK-47, but it may be preferable to switch to a sidearm for the Combat Rifle.

The AK-47 has the slowest rate of fire at just under 7.7 rounds per second. Without letting go of the trigger, it will take 5.2 seconds to empty a magazine.
The M-16 Assault Rifle has the highest rate of fire at 11.4 rounds per second. Without letting go of the trigger, it will take 4.4 seconds to empty a magazine.
The Combat Rifle fires in three-round bursts with momentary pauses between each burst. One burst and pause lasts 0.36 seconds. 2.79 bursts can be fired per second, and it will take 20 bursts over the course of 7.16 seconds to empty a magazine.
*All numbers are approximate

The AK-47 deals 58 damage per bullet at 446 damage per second. One bullet is enough to kill a single Common Infected on Easy or Normal Difficulty, while two bullets are needed for Advanced and Expert. It has the widest bullet spread when standing or moving as well as powerful recoil, making it the least accurate of the Automatic Rifles.
The M-16 Assault Rifle deals 33 damage per bullet at 376.2 damage per second. Two bullets are required to kill a Common Infected on Easy or Normal Difficulty, and three are needed for Advanced and Expert. It has moderate accuracy and recoil, making it the middle-of-the-road for the Automatic Rifles.
The Combat Rifle deals 44 damage per bullet at 368.4 damage per second. A three-round burst is enough to kill a Common Infected on any Difficulty outside of Realism. It is slightly more accurate than the M-16 Assault Rifle, and its momentary pauses between bursts allows it to regain some of its lost accuracy when holding the trigger down.

Laser Sights make the Combat Rifle a dangerous long-range burst weapon and improve upon the M-16 Assault Rifle's middle-of-the-road accuracy. The AK-47's atrociously wide spread is tightened considerably, making it capable of mid-range combat while running.
All images were taken at 1600x900 Resolution, Full-Screen, with Graphical Settings at maximum and Film Grain turned off.

Each Automatic Rifle has a unique magazine size. The Combat Rifle boasts a full 60 rounds, the M-16 Assault Rifle contains 50, and the AK-47 holds a comparatively measly 40. The Ammo Reserve is identical for each Rifle at 360 rounds. This allots 7 complete magazines for the Combat Rifle, 10 for the AK-47, and an awkward 8.2 for the M-16 Assault Rifle, including the initially loaded magazine.

Conserving your Special Ammunition by firing only one bullet at a time is easiest with the AK-47's low rate of fire. The Combat Rifle has the largest magazine for Special Ammunition, but the mandatory burst fire can cause it to run out of ammo just as quickly as the others.
Incendiary Ammunition can allow for greater ammo conservation. A Common Infected that would normally take two or three shots to kill can now be taken out in just one. Riot Cop Infected are impervious to bullet damage from the front, but vulnerable to Incendiary Ammunition from all angles, allowing you to dispatch them just as easily as any other Common Infected. Ignited Infected may obscure your vision, as it is harder to see past burning, flailing bodies to know just how many more Infected are charging towards you and your team. Five seconds worth of burn damage will be inflicted against any struck Special Infected, as well as making them more obvious to your group. Incendiary Ammunition is best for conserving ammo against a Horde and assaulting Tanks.
Explosive Ammunition can tear apart a closely knit group of Common Infected, transforming them into spare limbs and bisected bodies. Controlled bursts spread across tight groups of Infected is an excellent way to maximize your damage against a Horde. Explosive Ammunition also stumbles Special Infected, knocking them off pinned Survivors from a distance. This is one of the few ways to knock an offending Charger off an ally, as well as the safest way to do so for both you and your team. Don't stand too close to your target, though. You can damage and even stumble yourself or your allies with Explosive Ammunition. Explosive Ammunition is best for dealing with Hordes or rescuing pinned allies.

Combat Rifle users will notice that the mandatory burst fire of the weapon is overkill against a single Common Infected while using Special Ammunition. It is best conserved for Special Infected or Hordes. Turning and spreading your fire mid-burst can help minimize wasted ammunition.

Although the Combat Rifle, AK-47, and M-16 Assault Rifle can easily subdue most Infected with rapid fire, they are unique and can serve different gameplay styles and preferences.

The M-16 Assault Rifle has decent moving accuracy with a higher rate of fire than the AK-47. M-16 Assault Rifles may be preferred for Gauntlet Crescendos and Finales that force your team to keep moving while the Infected continually spawn. When covering a teammate, short bursts of fire while standing still is your best bet to take out the Infected without harming other Survivors.
The Combat Rifle's higher accuracy and controlled rate of fire allows it to pick off Infected at a considerable distance. Target prioritization is key. A Combat Rifle can more easily take out a Smoker on a roof or a Special Infected trying to blend in with a Horde. The higher accuracy makes it easier to cover an ally without accidentally shooting them, and the controlled burst deals just enough damage to kill any Common Infected outside of Realism.
The AK-47 has terrible moving accuracy, but high damage. It supports a stand-still defensive strategy, and may be preferred for Classic Crescendos and Finales that involve holding off against a Horde. Firing bullets one at a time helps to maintain accuracy and prevent ammunition from being wasted. The low accuracy and high recoil may cause you to accidentally shoot your teammates when providing cover fire.

The high damage output of the AK-47 and rapid fire of the M-16 Assault Rifle make them preferable for dealing with Hordes of Common Infected. The Combat Rifle's calculated rate of fire and damage is better for picking off stragglers that separate themselves from the Horde or try to flank your team.

Smokers tend to keep back at a greater distance than other Infected, making it tougher to hit and kill one with a low accuracy weapon. The M-16 Assault Rifle offsets its lower accuracy with rapid fire, while the higher accuracy and controlled bursts of the Combat Rifle make it ideal for dealing with Smokers before they snag a Survivor. Aiming for the head or upper chest and releasing one or two bursts should at least frighten a Smoker and cause it to retreat, if it is not killed outright.

The rapid fire of the Automatic Rifles coupled with their magazine-based reloads make them effective weapons for handling Tanks at a moderate distance. The AK-47's low accuracy may make it less effective at greater ranges, but the Tank's hulking figure makes up for this as he gets closer. Some Automatic Rifle users may prefer to fire at the Tank while standing still and/or crouching to improve accuracy, only moving to avoid or kite the Tank and its attacks.

Witches are slim, quick, and have high health. Automatic Rifles are better reserved for protecting other Survivors from Witches, as the accuracy penalty from running and shooting drastically decreases the number of bullets that will hit a Witch chasing you. If you do startle a Witch, get to high ground where the Witch will have to climb to reach you. This puts her in a vulnerable position where your team can easily shoot her down.

Hunting Rifle vs. Sniper Rifle
Combat Shotgun vs. Tactical Shotgun