Pump Shotgun

The pump-action shotgun deals heavy damage to an enemy or lighter damage to many, in a spread. It holds 8 shells with 128 in reserve. The spread of the 10 pellets in a shell can take down multiple Infected at a time. This often makes it better at handling hordes than the Submachine gun or Pistols. It reloads about 2 shells per second and fires 1 shell per second. The spread of the shotgun makes pistols better than it for ranged encounters.

All shotguns allow topping off the magazine on the fly, and the reloading process can be stopped to handle sudden threats. However, if you are reloading when empty, you will manually pump the shotgun at the end of the reload, regardless or whether it finished or was interrupted. A good trick is to interrupt an empty reload after the first shot by clicking the fire button and then reloading again, as you have gotten the racking animation out of the way and can immediately fire if a threat appears. Another tip is just to reload constantly and always keep at least one shell loaded.

Like the Auto Shotgun and Combat Shotgun, the Pump Shotgun can kill the Witch with one hit if all the pellets strike her head before she rises, making it useful for getting the Cr0wnd Achievement.

Tactics

 * While being attacked by hordes, it is best to do a melee attack after each shot to push back Infected while pumping, to increase survivability.
 * You can melee as you reload instead of just standing there, waiting to finish reloading. This helps with keeping the Infected at bay.
 * Team members with shotguns should be at the front to reduce the chance of friendly fire.
 * If you're trying to help someone who is surrounded on any difficulty levels aside from Easy or Normal, switch to the Pistol. If you try to use the shotgun, you're most likely going to end up dealing more damage than the Infected themselves. If you must use a shotgun to rescue a surrounded teammate, keep the crosshair off the teammate to kill Infected with the spread.
 * When attempting to headshot a Witch its best to have the top line on the crosshair aimed for the Witch's forehead. This tactic, however, only works about 85% of the time.
 * If your teammate is being attacked on the other side of a wall, you can use the shotgun to shoot through the wall to save them.

Left 4 Dead 2
The Pump Shotgun returns, but has a new model, a larger spread, and does more damage per pellet. Compared to the Chrome Shotgun, it has a larger DPS due to 2 more pellets per shell (250 damage/s (If every pellet hits), 4 more than its Chrome counter-part) but a larger pellet-spread.

Behind the Scenes
An unusual ammo counter for the pump shotgun's magazine is in the game's files. It shows eight shotgun shells. There are two versions; one for an empty magazine and one for a full one. This has never been seen in any footage, which means that it is much earlier than the footage shown. This seems to hint that in very early versions of the game, the player ammo count would be shown with a physical representation of how much ammo is left in the magazine. It has a resemblance with Medal of Honor : Allied Assault ammo counter.

The Pump Shotgun is based on Counter-Strike ' s M3 pump-action shotgun. Its "price" in the gun's weapon file is the same as the M3's, and it references the M3 in its "animation" listing. The M3's firing sound is played when Francis fires the pump shotgun in the first Left 4 Dead teaser, released in January 2007.

Pros

 * Extremely powerful early in the game, capable of going though hordes of zombies.
 * A lot more powerful at close range when compared to the Submachine Gun in L4D.
 * Due to low rate of fire, it is easy to control and doesn't run out of ammo nearly as fast as the other guns.
 * Can take out Special Infected faster than the SMG at close range, especially the Hunter.
 * Low rate of fire means it can reduce friendly fire.
 * Deadly at close range.
 * Spray of the fire means it can take out multiple common infected at once.
 * Great for tight, or narrow spaces like choke points in buildings.

Cons

 * Each shell in the gun must be reloaded manually so it has one of the longest reloading times in the game, although the reloading can be interrupted to fire the pump shotgun.
 * Extremely inaccurate at long distances due to the large spread, even the pistols are more effective.
 * Strikingly less effective against the tank and other SI with high health at medium to long range.
 * Low rate of fire means its difficult to hit a fast moving target.
 * When grabbed by a smoker at long range its nearly impossible to kill him in time by yourself before you're dragged off.
 * On Expert, friendly fire becomes an issue. At close range, one shot can incapacitate a teammate.