At the BBOD we deal almost entirely in sporting grade shotguns, but we also sell a few personal defense guns, so let's talk about them for a minute...
The personal defense shotgun is probably the most often thought of, but least understood firearm for defensive and offensive use. We can start by defining the personal defense shotgun, what is the PD scattergun? The one you have on hand or the one you buy to use to protect yourself. Barrel length is not the criteria, let me repeat that, barrel length is not the criteria, you need to be concerned with. What you need in a respected defensive gun is reliability and the ability to hit what you are aiming at.
With that said, let's talk about preferred options of the defensive shotgun:
-Short(er) Barrel, generally 22" in length or less, typically an 18.5" or 20" barrel is found. Personal defense guns come in all shapes and sizes, but the guns that are designed and marketed as such are found usually with a 20" or shorter barrel length. The shorter barrel allows you to wield the gun more effectively inside your home, especially if you live in a smaller or more confined space. I recommend the longest barrel you feel you can effectively swing and get through doors for house clearing (I do NOT advocate house clearing, but if it has to be done, do it with your long gun!). The longer the barrel, the longer the sight radius, the easier it is to aim and hit with. Also, many of the defensive guns on the market are fixed cylinder choke barrels, if you can purchase a gun with screw in chokes. Screw in chokes will increase the versatility of your shotgun exponentially. Fixed Cylinder works okay with some buckshot loads and some slugs, but you need to experiment with loads and patterning to find the most accurate. Being able to adjust the choke, might allow an even tighter pattern or more general accuracy from your chosen ammo.
-Safety you can use; this one just makes sense, right? Yet, so many buyers out there buy shotguns and figure they will leave them in "Cruiser Ready" mode, or fully loaded with a round not chambered and the safety set. Guess what? If you can't work the safety, you're going to be in a world of chambered with a loaded shotgun that still doesn't work. When milliseconds count and you are fighting for your life, don't try and remember where the safety is and how it works, know it. I'm an advocate of the top tang Mossberg-style safety, because it works universally right and left hand and is easy to use and see, with a traditional stock. That said, there is nothing wrong with the position of the Remington safety for a right hander and many companies make left handed safeties for the Remington series of guns.
-Controls you can reach without having to adjust your grip drastically (I.E. can you reach the slide lock without having to take your main shooting hand off the gun?) This goes back to the safety argument above, if you are running a pump, can you hit the slide lock with your off hand, what about one handed? Again the Mossberg camp wins this with their behind the trigger guard on the left setup, versus the Remington in front of the trigger guard setup. How about the semi-auto guys, can you hit your bolt release or bolt lock back button without sacrificing control of the gun? It's critical, weapon control is as critical as hitting your target. Maintain it and pick a gun that has controls where you like them and where you feel like they should be.
-Semi-Auto vs. Pump vs. Non-Reciprocating Action Guns: This debate will rage until the end of time. Lets get down and dirty and quick. The Mossberg 500 and 590 guns, the Remington 870, the Benelli Nova and SuperNova, are all exceptionally reliable and functional pump shotguns. With a pump you do run the risk of short stroking the gun. If you are concerned about this, my suggestion is to buy a Remington. The Mossberg and Benelli guns are smooth, but not the same level as the 870 series of guns. The smoother gun will be easier to run and generally result in fewer operator errors, but the only way to be truly good is to practice. Semi-auto, gas guns will have a reduced felt recoil to pump guns and inertia driven guns, gas guns typically are more picky about loads, a pump will generally feed and shoot anything that fits in the chamber. Inertia Driven semi-auto guns, the Benellis, Berettas, and Stoeger guns are all super cool guns, but just not what I think it takes for personal defense. The Inertia Driven system is a super recoiling system and limits the ability to make fast follow up shots with defensive type shot loads. Non-reciprocating action guns, single shots are cheaper, but not necessarily any better than a quality pump gun, if it's what you have, learn it and love it, if you are in the market to buy, go pump if you can.
-12 or 20 gauge; another great debate. 12-gauge 00 buckshot has the record, 000 might be even better. 20-gauge has #3 buck in 2 3/4" shells and #2 in 3" shells, in my opinion, opt for 2 3/4" shells, the recoil vs. increased amount of shot or shot size, is not worth the trade off. It doesn't matter which you choose if you pick a good quality buckshot load AND you learn to aim.
-Ghost Ring Sights vs. Bead vs. Fiber Optic, now the truly important bit on this subject, sights. Gun sights are there for a reason, racking the shotgun and pointing in the general direction will not guarantee hits with the shots. You need to aim and the best way to do it, is to use the sights. Ghost Ring or Rifle type sights are all the rage these days with many of the "tactical" shooters and while I generally agree that Ghost Rings are faster and easier to focus on, I don't necessarily agree that they are the only choice in shotgun sights. A bead sight and vented barrel rib works well for tracking small, quickly flying game like dove, it can work equally well on a shorter barrel, for tracking a much larger, but still quick, human target. In this case, the Ghost Ring sights are advantageous in they help the shooter focus on the front sight and aim, but they are not the end all be all. If you have them on your gun, love them, learn them, shoot them, if you don't, spend your money on buckshot and range time.
Now, fiber optic sights, many new guns have them, especially sporting guns, bright red and green inserts certainly allow you to spot the right sight and keep a good eye on things. I personally, really like the Tru-Glo setups that are meant to attach over your standard bead sight and allow you to see a fiber optic tube. I prefer this setup to a fiber optic only setup, because the fiber rods do occasionally fall out. If that were to happen to you, you'd at least still have a front bead to focus on and shoot from.
-Other Accessories like butt cuffs, side saddles, and flashlights; Spare ammunition is good, having it on the gun is good, because we will probably fight with the ammo in and on the gun, but there is too much of a good thing. Choose one system and use it, use it, use it, until it is second nature to reload your shotgun from the butt cuff or side saddle. Personally, I opt for butt cuff, because I'm on a low budget and side saddles don't ever seem to come cheap. The stretchy spandex like butt cuff that holds five shot shells will fill all your needs. You do need to do one thing, secure it to the stock. I did this, by using marine grade velcro, the hook side on the cuff and the loop side on the stock, held on by marine grade waterproof adhesive. The cuff does not move around on my gun and allows me a platform to use.
On flashlights, just do it. You don't have to go for a Surefire flashlight fore end, but there are many setups out there on the market. Find one that works for you, use it, learn it, love it. Personally, my setup is pretty redneck, opting for a positive 'clicky' tail-capped LED Mini-Mag Light, that is secured to the fore-end via duct tape and more marine grade velcro. It's very ugly, but extremely functional, it does not effect the ability to use the fore-end nor does it move around while the gun is in use. Again, pick a system and practice with it, until it is second nature.
~Whew~ Let me take a breath, are you confused yet? Well, lets get back to basics. For personal defense, the shotgun that is for you, is the one with controls that you are familiar with and feel comfortable with. Your next big issue is picking a gauge that works for you, if you are recoil sensitive, don't buy the hype that 20 feels less than 12, because many 20-gauge guns are also lighter in overall weight. The lighter the gun, the more felt recoil, period, in this case, if the guns weigh the same, yes 20 will have a little lighter recoil, but 12-gauge will still be managable. Next, you need sights you can use and perhaps you should consider spare ammo and a flashlight on your gun. The later two items can help in a fight but aren't necessary, the sights are. Pick a sighting system that allows you to aim and hit. Finally, whatever you pick, practice, practice, practice, and train, train, train. The shotgun might be an extremely powerful and versatile tool for personal defense, but you need the ability to wield that power.
Defensive shotguns are extremely useful and should be part of everyone's toolkit. Selecting a gun is not hard, pick the gun that fills your needs with the least amount of compromise and practice with it. For those wondering about my top picks for defensive shotguns the list is as follows: Remington 870, particularly the Express Series "Defense" Models in 12-gauge, the Mossberg 500 and 590 in 12-gauge, the Remington Express 870 20-gauge "Youth" gun, the Mossberg 500 20-gauge "Youth" gun, and then the Benelli series of guns. The Benellis come last, because parts for the Mossberg and Remington series of guns are literally a dime a dozen of anything you need.