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. Although the range on the shotgun is much better than most First Person Shooter games and truer to reality, the spread of the shotgun makes pistols better for ranged encounters.

On both shotguns, the reloading is both a curse and a blessing; it allows 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 simply never have less than 2 shots in your shotgun at any one time, reload constantly.

Like the auto shotgun, the 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.
 * Also, you can melee as you reload instead of just standing there, waiting to finish reloading. This helps tremendously at keeping the Infected at bay, and dealing minimum damage that can help to kill the Infected faster.
 * As a rule of thumb, team members with the shotgun should be at the front to reduce the chance of friendly fire.
 * This is fairly obvious, but do not use the shotgun as a sniper rifle of sorts unless you're playing on Easy.
 * 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. It's also useful if your teammates are inevitably surrounded on all sides, to use your shotgun on the Infected around them. To do this your cursor must never be on your teammate however.
 * 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.
 * The shotgun can be used to snipe and take out a few zombies from long range if you use all the bullets in one clip. (Confirmed)

Left 4 Dead 2
The pump shotgun has a new model in Left 4 Dead 2. It appears to be a Mossberg 500 Mariner, originally designed for marines because the stainless steel build doesn't rust or rot like a normal steel or wooden build would; a constant issue on boats and ships where the salty air causes such problems. This is probably why it is is a pickup in Left 4 Dead 2, which is set in the south as traversing wet swamps would come with similar problems.

Trivia

 * The side of the shotgun says "Renegade Rangemaster", which is meant to sound similar to the very common "Remington Wingmaster", a variant of the Remington 870. The safety and slide release resemble the kind seen on the Remington series shotguns, but it possesses several features found on Mossberg shotguns, notably the 500 Mariner: The pump foregrip is ribbed, where Remington shotguns nearly always use a smooth foregrip. The magazine tube holds 6 rounds and extends one round beyond the pump; most Remington shotguns either have the magazine tube run all the way to the end of the barrel, or just the end of the foregrip. Likely, Valve decided to mix features to give the shotgun a look that retained the distinct look of a hunting shotgun, while not looking totally generic. This is similar to what the developers did with the L4D 1 pump shotgun. A flashlight is secured to the shotgun by a magazine tube mount. Also, the magazine tube cap is more like the Ithaca M37 shotgun on closer inspection.
 * 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.
 * Valve recycled sounds from Team Fortress 2 ' s pump shotgun and used it for Left 4 Dead ' s pump and auto shotgun.
 * The shotgun is reloaded incorrectly by the Survivors, when doing an empty reload the Survivors end the reload by pumping the shotgun, which adds an extra space in the shotgun's tube magazine, yet the Survivors don't put the final round in.