I settled years ago on my favorite type of defense gun. It is the riot gun, 12 gauge. I refer to the repeater, either pump or semi automatic, not Joe Biden's silly double barrel. The double is a fine gun for dealing with wabbitty wascallity, but let's be real here. If you "Buy a shotgun, buy a shotgun," make sure it holds plenty of ammo. Buy a riot shotgun.
Since nearly all justifiable self defense shootings are at short range, it makes sense to choose the best short range gun. This is it, beyond doubt.
That's a big claim I made. Want proof? I have several points for you to ponder. A JSSAP study rated the shotgun as having twice the hit probability of an assault rifle and nearly half again better than a submachine gun. Furthermore, the word on the street is that someone put down with "the gauge" is generally down for the long count. Criminals fear this gun, for it has done away with many of their friends and relatives. For most of the last century, the 12 gauge pump was the flatfoot's friend, the gun most likely to be in a cop's hands if he whiffed trouble ahead of time. As for the cops' current defection to the glamorous M4 carbine, it is a fickle phase. They will be back to their old girl before long. Twice the probability of scoring...
The lightest body armor now in use will stop even the best buckshot. That was not a problem years ago, but now it is. Formerly a bad guy with body armor was a very rare occurrence. Criminals these days are getting wise to the advantages of a bulletproof torso. But the armor does not cover everything. The shotgun's hit probability, which owes to the spread of the shot, makes it possible to target and hit body parts not covered by the armor, such as necks and knees.
For this kind of shooting, your chances are improved by using smaller shot than the famous 00. 00 does not have much pattern density, so we need to think smart. #4 buckshot (never to be confused with #4 birdshot, which is many times too small for defensive use) gives three times the pattern density of 00. That is to say, the common loading carries 27 pellets to 00's nine pellets. So then, within a given area within the shotgun's pattern, the likelihood is that there will be three times as many impacts. #4 buck's reputation is that it sometimes produces insufficient penetration on torso shots. But so what? We aren't shooting at the torso anyway. That's bulletproof.
#1 buckshot (never to be confused, etcetera) is a compromise between the good range and penetration of 00 and the good pattern density of #4 buck. Federal Cartridge is offering #1 buck with their excellent FliteControl wad. It is a reduced recoil load, 15 pellets, at reduced velocity, about 1100 fps. This loading has not been available long enough to have much of a track record, but it appears to be an improvement, for present day circumstances, over reduced recoil 00; it has better pattern density and nearly as much penetration. The extra pattern density is helpful if you need to make the difficult shot that hits the attacker where his armor does not protect him. My only reservation about this stuff, which also applies to the other "super wad" tight-patterning buckshot loads (Federal FliteControl, Hornady VersaTite) is that in some situations the patterns will be smaller than optimum. Of course we give up some of the shotgun's outstanding hit probability when we halve the size of the pattern. On the other hand, the gun is effective at longer range. I'm watching with interest to see the practical, street level consequences of the super wads. I'm sure that for some uses they are an improvement, but sometimes a wide, fast-opening pattern can be a life-saving advantage.
Thus my view of the matter. In the near distance engagements that are typical for personal defense situations, I would rather have a riot shotgun than any other small arm. I want to be the one explaining what happened.