Well there is some pretty good advice so far, and some "not so good" opinion here. I will agree with some of it, and disagree with some of it, but I will speak from FIRST HAND EXPERIENCE both professionally, and in the field having INVESTIGATED, AND VIEWED RESULTS of the shotgun, not only in SELF DEFENSE, but OFFENSE. LEGALLY AND OTHER WISE, over a 20 year career.
As a L.E. weapons instructor for 18 years, several years a SWAT team leader, and instructor, I have used, and seen used the shotgun. I used to teach a course called SHOTGUN STRESS, which dealt with the many different methods a shotgun could be used in a fight. AND there are many.
Weapon: Both the Mossberg 500, and the Remington 870 are excellent choices, with my PERSONAL choice being the 870. I personally use a modified Remington 1100. No lights, no picatinny mounts on the forearm, but a strip down pump shotgun, with a corncob forearm. KISS!
Loads: I do not recommend anything OVER 2 3/4" shells, as a short pump, and yes, even in an auto loader, may cause a hull NOT to clear the ejection port. In buck shot I recommend 2 3/4" MAGNUM in #4 "buckshot" for anything "outside" the house. A 2 3/4" mag load of 00 buckshot has 9 pellets. I have seen a large dog (pitbull) more than once, at fairly close range run thru a pattern of 00 with out a scratch. #4 buckshot in the same load I believe is 27 pellets. I am going by memory here.
If a DEFENDER has family INSIDE THE HOUSE, he or she does not want to shoot them, and here is where I will no doubt get resistance. Let me say, I do not care, I have WORKED ACTUAL SHOOTINGS, I AM NOT GUESSING, OR GIVING AN OPINION.
Inside the house 7 1/2 low base BIRDSHOT, is more than enough. It spreads very little at room distances, doesn't reliably penetrate both wall surfaces, and is less dangerous to FAMILY MEMBERS IN OTHER ROOMS. If you have family in the house, YOU SHOULD NEVER SHOOT THRU WALLS INTENTIONALLY, AS YOU CANNOT KNOW WHOM YOUR SHOOTING AT.
98% of all NON POLICE shootings I worked with a shotgun involved, were done with BIRDSHOT. 100% of ALL INSIDE RESIDENCE shots with these shotguns were FATAL, if the shot was center mass FATAL AT THE SCENE.
I once had an emergency room doctor tell me that he wished they would outlaw folks from shooting each other with BIRDSHOT. I asked why, and he said: I can't fix HAMBURGER.
Now! With that said, I know that at least one here will disagree, and as I said, I don't care. I have actually "BEEN THERE AND DONE THAT".
It isn't required that anyone take my word for anything I said, I gave good advice based on a lot of experience and training, and I have no intention of debating it.
Hope I helped,
Dan, aka Dee