1. Effectiveness at short distances - Shotguns (and in this discussion, I am always talking about 12 gauge shotguns shooting buckshot) are very effective at "in the home" distances. As Clint Smith always reminds us, "At short distances, shotguns will remove meat and bone".
Past 30 yards, they become less effective without a different load (slugs), but for inside the home, they are very effective.
2. Cost - An AR15 carbine will start at about $700 and go up to several thousand dollars, depending on how many bells and whistles it has on it.
A very functional 12 gauge pump can be found for $150 or less.
Nearly anyone can scrape up the funds for a shotgun.
3. Intimidation factor - Not to over-state this one, but I when I was a police officer, I saw that a shotgun commanded a lot more respect than a handgun. Maybe justified, maybe not, but there it is.
And no doubt about it, a shotgun is much more powerful and effective than the typical centerfire handgun.
1. It has low ammunition capacity, requiring more "manipulation", which is a bad thing. Carbines can have 30 round magazines, therefore requiring much less manipulation.
2. Heavy recoil - Some folks have a hard time dealing with the heavy recoil of a shotgun. This can be dealt with in many ways, but it will always kick more than a carbine.
3. Shot spread - Buckshot tends to spread from an open bore at about 1 inch per yard. At longer ranges, some pellets may miss the bad guy. Where will they go? You had better know, as you are responsible for every pellet.
Carbines allow much better "accuracy" which might be an advantage in some scenarios.
4. Difficulty of manipulation - Some actions, such as pumps, can be difficult for some to operate. And anyone that has experienced a "short stroke jam" with an 870, knows what I am talking about.
This can be overcome with training, but many do not have the time or desire to devote the time required to master this problem.
5. Poor design - Many variations of shotguns, such as those with a pistol grip and no buttstock, are horrible to use. Of course this is easy to fix, but it is a disadvantage for those that do not know better.
6. Weight - A shotgun loaded with a full load of 5 to 8 rounds in the magazine and a carrier with 6 rounds in it, is a heavy weapon.
Basic Shotgun Requirements
I suggest the basic shotgun used for home defense should have the following characteristics:
1. It should be a 12 gauge.
A 20 gauge can be effective with the proper ammunition, but you don't get something for nothing and it will not be as effective as a 12 gauge.
2. It should be loaded with Buckshot.
Any buckshot will reach the FBI required minimum penetration of 12 inches, but studies seem to suggest that #1 or 00 buckshot are best.
Birdshot is only for practice or shooting little birds.