Unlike the pump-action shotguns which require the user to manually operate the action before each shot, semiautomatic models load and cock the action automatically. A small lever is operated to initially load a shell into the chamber of the gun, but after that the gun reloads and cocks itself after each shot. This makes a semiautomatic shotgun much faster to shoot.
The downside of using a semiautomatic shotgun is that you need to remember to engage the “safety” once you are done shooting. This is because if the trigger is pulled again, or snagged by clothing or a tree branch, it will automatically fire again.
Another disadvantage is that a quality semiautomatic shotgun will be much more expensive to purchase than a pump-action gun. I don’t recommend using a less-expensive semiautomatic as they have a tendency to jam at the most inopportune times. You can’t risk this problem when using a tactical shotgun for self-defense, so if you want an autoloader, you need to step-up and buy a good one.
In addition to faster operating speed, another immensely positive aspect of a high quality semiautomatic shotgun is reduced recoil. This reduced recoil is modest in some semi-auto models and substantial in others. For example, when compared to a standard pump gun such as the Mossberg 500, the Benelli M2 Tactical with ComforTech stock literally cuts perceived recoil in half. This is a huge difference.
Reduced recoil it is a benefit which may be important for those who are physically small or have reduced mobility, for those who are intimidated by recoil, and for those who want to be able to fire their gun more rapidly. Reduced-recoil combined with a semiautomatic action, makes it possible for the user to keep the gun on target between shots.
The Benelli M4 is currently the gold-standard for SWAT teams and military units because it is easy to use, accurate, reliable, and amazingly fast. And I do mean FAST. It’s like using a submachine gun – but better. The Benelli recoil-reduction system means you can keep the M2 or M4 on target even during rapid fire. http://www.flickr.com/photos/52451337@N00/3596407349/
The Benelli Super-90 is an older model and excellent, but now hard to find. I’ve enjoyed demonstrations using the Benelli Super-90, holding it in one hand, like a pistol, and hammering the target rapid-fire. This is only possible because the gun itself absorbs most of the recoil. The Super-90 has now been superseded by the M2 model, but both are exceptional guns. A Benelli M2 equipped with a ComforTech stock has far less recoil than the Benelli M4.
For most people, before opting for expensive add-ons such as a collapsible stock, that money would be better spent on acquiring a top-notch semiautomatic rather than a pump-action model. Spec Ops military units generally use the Benelli M4 Tactical, and premier SWAT teams may also use the Benelli M2 model, but the Beretta TX4 Storm is now giving the Benelli some competition. The cost of these semiautomatics is much more than a good pump shotgun, but these are certainly impressive weapons which have far less recoil: http://www.benelliusa.com/shotguns/benelli_m4.php. The advantage of the Beretta TX4 Storm is that it seems to feed low-recoil ammunition, whereas it’s best to use either standard or magnum loads in the Benelli models.