I'm going to ruffle a few feathers with this one.
For defensive use a shotgun is a short ranged heavy recoiling rifle with a small magazine capacity and bulky, heavy ammunition.
There. Now for a few details.
If you are going to use a buckshot loaded shotgun for defense you need to take that thing out and pattern it WITH THE LOAD YOU WILL ACTUALLY BE USING from contact distance out until the pattern spreads so wide that you aren't getting enough pellets into the target to be effective. Various loads pattern differently in different shotguns and the some spread faster than others. Just because your shotgun will keep all the pellets from a Winchester 2 3/4 inch 00 load in a 10" circle at 10 yards doesn't mean that it will keep all the pellets from a Federal 2 3/4 inch 00 load in a 15" circle at the same distance.
You need to know what your shotgun will do with your load at every possible distance. The cheaper loads you usually find at Walmart often have the most rapid spread. I tried some of those bottom end Winchester and Federal 00, #1, and #4 loads from Walmart in four different high quality cylinder bore shotguns. Almost all of the loads kept all the pellets on an 8.5" x11" paper at 10 yards. By 12 yards all were missing that paper with some pellets. By 15 yards the patterns overall were about 50%. That means the 00 buckshot you usually find at Walmart would keep 4 or 5 pellets on an 8.5" x 11" paper at 15 long paces. Many people have shots longer than that inside their house. The higher dollar loads will usually pattern better and an improved cylinder or even modified choke tube could help, but there is no guarantee. If you are going to use a shotgun then pattern your gun with your load. I also tried those four shotguns with cheapo Foster slug loads. The three with ghost ring sights would keep all their slugs on an 8.5" x 11" paper out to 100 yards. At 25 yards all would easily keep all their slugs in a group you could cover with the palm of your hand. I didn't bother trying any of them with cheap buckshot loads at 25 yards. Why do I keep bringing up cheap loads from Walmart? Because if you ever run short in an emergency situation you probably won't have time to order that special load from somewhere online and you'll just run down to the nearest Walmart or Academy and buy whatever they have on the shelf. I have seen some shotguns with high dollar loads that would keep all the pellets of that particular load in a 10" circle out to 25 yards and one that would keep all the pellets in a 10" circle at 35 yards so it can be done but it may take trying a number of different loads and choke tubes to find one that works that well.
My advice is to take a defensive shotgun course from a good school. Then take a defensive rifle course from a good school. More than one of each if you can afford it. My experience is that the exercises you do in the shotgun course would be easy with a rifle in 5.56x45mm or 7.62x39mm and no more difficult with a .30-30 levergun or a 7.62x51mm semiauto than they are with a shotgun. The exercises in the rifle course will often be very difficult with a shotgun even if you exclude anything outside of 50 yards.
One of the things you will probably be exposed to in a defensive shotgun course is a "select slug" drill. The idea is that during a fight with people shooting at you, if you suddenly see a target 20+ yards away (outside buckshot range) you should take the buckshot round out of the chamber and substitute a slug which you then use against the distant target. Does that sound like a prudent thing to do? It doesn't to me. I took a defensive shotgun course with Louis Awerbuck and we did some of these. There are a variety of methods to use depending on the type of action your shotgun has and how lucky you are feeling that day. One thing they all had in common was that they all sucked. All were at best tedious, complicated, time consuming, and tended to get your attention focused between your hands when it should be out there where the threat is. At worst they were suicidal. Long before that class I thought the right thing to do with a defensive shotgun was to load it with slugs and I came out of the class more convinced of that than ever.
If you are going to load your shotgun with slugs then why not just use a rifle instead?
BTW during the class someone asked Mr. Awerbuck what he used for home defense. His answer was that he kept 3 or 4 shotguns scattered around his house. They were all identical pump shotguns with rifle sights and all were loaded exclusively with slugs. Why loaded with slugs? Because he didn't want to have to worry about which gun he picked up and how it patterned at each range. With all of them loaded with slugs and all zeroed the same they will all hit the same inside of 25 yards. They will probably all hit pretty much the same out to 100 yards.
I wouldn't advice trying to shuffle the round in the chamber in the middle of a fight. The best method for me was to shove a slug into the magazine then run the action and lose the round that was in the chamber. That wastes ammo and of course won't work if the magazine is full. Some semiauto shotguns may give you trouble with that method as well.
The biggest problem with the defensive use of shotguns other than the short effective range is that many people think they are an ultimate weapon that requires no knowledge or skill to be effective. "I'll scare them off with the sound of it being racked . . ", "I don't even have to aim . . . ", "I'll only need one shot . . . ", and all that kind of nonsense.
Me? I'm not that worried about the creeps that can be scared off with a sound - I'm worried about the ones that can't be scared off at all. I'd rather have two 20 or 30 round mags than to have a 5 round mag and 5 or 10 more rounds strapped to the gun. I don't plan on missing but I also don't plan on shooting as well under real stress as I do in practice and while I don't expect to face a zombie horde I don't have the power to see into the future so I really don't know what I'll face. I also don't count on one round ever being enough.
Use a shotgun if you want but make sure you understand the limitations.
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