While playing Left 4 Dead 2 with friends or on public servers, I have noticed that many players are unaware of the differences between the Tier 2 Shotguns. Some can't even identify the Shotguns by name, further confusing themselves and others as to which Shotgun does what.
The Combat Shotgun is based on the Franchi SPAS-12, while the Tactical Shotgun is based on the Benelli M1014. I have provided an image below with both Shotguns clearly labeled.

Both the Combat Shotgun and Tactical Shotgun are fully automatic in-game. The user can simply hold the trigger to fire at a steady rate, or tap their trigger button to fire at their own pace.

Both Shotguns reload shells one at a time. The player can stop reloading at any point to begin firing again. For the Tactical Shotgun, the first shell of a reload session takes more time to be loaded than every shell afterwards, meaning it is faster to load many shells in a row rather than one at a time. This is not the case for the Combat Shotgun. It reloads all shells at the same speed, making constant firing and reloading a more viable strategy. When the player finishes reloading, they will pull the lever on the Shotgun. This is purely aesthetic and the animation can be stopped and the Shotgun fired at any moment.

Both the Combat Shotgun and Tactical Shotgun deal just over 250 damage per shell.
The Combat Shotgun features 9 pellets per shell with each pellet dealing 28 damage. It has a slightly tighter spread than the Tactical Shotgun, affording each pellet a greater chance of hitting the intended target or targets, as well as increasing the distance at which the weapon is still effective.
The Tactical Shotgun features 11 pellets per shell with each pellet dealing 23 damage. It has a wider spread than the Combat Shotgun, granting increased breadth of fire for less tightly knit groups of Infected, but suffers at mid-range or beyond.

Laser Sights have very little effect on the accuracy of either Shotgun. The Tactical Shotgun is made as accurate as a Laser-less Combat Shotgun, and the Combat Shotgun's pellet spread is tightened ever so slightly. Laser Sights also allow your teammates to know where you are aiming. This may be all it takes to prevent an ally from moving into the line of fire.


























All images were taken at 1600x900 Resolution, Full-Screen, with Graphical Settings at maximum and Film Grain turned off.

Both the Combat Shotgun and Tactical Shotgun can hold 10 shells at any given moment, and both feature an Ammo Reserve of 90 shells. This allows either Shotgun to be fully reloaded 9 times if the user waits until empty.

Seeing as both Shotguns hold 10 shells and dole out similar damage, there is no clear winner for using Special Ammunition.
Incendiary Ammunition allows the Shotguns' pellets to ignite Infected, making it possible to dispatch any Common Infected with just one pellet. This improves the ranged effectiveness of the Shotguns, as the player can kill any Common Infected as long as they can hit them. 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 applied to Tanks.
Explosive Ammunition replaces the Shotguns' standard buckshot with explosive slugs. The slug will land at, or very close to, the center of a Shotgun's spread. Explosive Ammunition can tear apart a closely knit group of Common Infected, and the accurate slug increases the ranged effectiveness of either Shotgun. 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 used against Hordes at a distance or stumbling Special Infected before they get too close.

Although the differences between the Combat Shotgun and Tactical Shotgun are very easy to overlook, they can support separate uses.

The Combat Shotgun has fewer, more damaging pellets in a more accurate spread. It is preferred for slightly greater ranges than the Tactical, and can be used to fend off Infected that have yet to move into closer ranges. Combat Shotguns may be preferred for Gauntlet Crescendos and Finales that force your team to keep moving while the Infected continually spawn.
The Tactical Shotgun has a larger pellet spread with more pellets, making it superior for close-range combat and for fighting against groups of Infected. Tactical Shotguns may be preferred for Classic Crescendos and Finales that involve holding off against a Horde.
When covering allies from Common Infected, it is best to place your cross-hairs to the side of your teammate, so you can harm the Infected with the pellet spread without accidentally shooting the ally you're trying to protect.

Shotguns have increased damage and penetration against Common Infected at close-range, making it easier to subdue nearby attackers. The pellet spread of a Shotgun makes it the preferred weapon for hunkering down in front of the group and fending off the horde as it closes in. Finding a location with protection on either side that funnels the Infected towards your Shotgun, such as a doorway or short hall, can improve your odds against a Horde. Using a Shotgun to pick off Common Infected one at a time can be a waste of ammunition, so a sidearm may be better for picking off stragglers. The wider spread and higher pellet count of the Tactical Shotgun provides a greater window for taking out as many Common Infected at once as possible.
A high damage, yet non-lethal Shotgun blast can cause a Common Infected to stumble. This is easier to observe with Riot Cop Infected, since they are immune to bullet damage from the front.

Every Shotgun has a 15% damage penalty against Tanks. Instead of dealing 250 damage, every Shotgun blast deals slightly over 210 damage. In spite of this, Shotguns are still highly effective against Tanks. The Tanks' size offsets much of the Shotguns' inaccuracy at mid-range. The Combat Shotgun's tighter spread and greater damage per pellet makes it a popular choice for fighting Tanks.

A player must get up close and personal with an as-of-yet unstartled Witch to cr0wn her with a Shotgun. Even if a Witch is not successfully cr0wned, the high damage output of the Shotgun can be enough to cover an unlucky ally or failed opportunity. The tighter spread of the Combat Shotgun makes it a popular choice for cr0wning.

Hunting Rifle vs. Sniper Rifle
Combat Rifle vs. AK-47 vs. M-16 Assault Rifle