OK, but just to get good info out there.....not to give you a hard time.To the OP, apparently I don't know what I'm talking about but no one wants to say why (except for the 870 bit). Good luck to you . . . and me. It doesn't matter -- shotguns are fun. You should have one.
"Here are my suggestions.
I suggest that your pistol is fine, but if you want to add a shotgun and know nothing about them, be practical. Get a semi-automatic gun (the 870 mentioned I think is semi-automatic) because it'll take some of the work out of the equation. A pump can be hard to manage -- I can't do it under calm conditions.
The 870 thing has already been cleared up, but another reason to use a pump has been reliability over the semis. I don't think this is valid anymore, but you will still hear it occasionally.
Get a "brush gun." That is, one with a shorter barrel like you would use if hunting in the brush, rather than shooting at geese high up in the air. Get an open choke, which simply means the barrel is more open or "bigger" at the muzzle so the shot spreads out over a shorter distance. Modified or closed choke -- hope I used the right terms -- are for distance shooting.
The shorter barrel is a very good idea, but is taken care of in a typical HD shotgun and the HDs have a higher capacity than a short barreled hunting weapon. How quickly the shot spreads in HD situation is irrelevant because of the close range. However, use of an open choke is still a good idea when shooting slugs (less stress on the barrel) and buckshot. A tight choke can actually cause the larger buckshot pellets to bounce off each other as they leave the barrel causing very erratic patters even at close range.
Get a 12-gauge because it's more powerful and more versatile. You can hunt anything with a 12-gauge from squirrel and rabbit to deer. For home defense it'll give you the most power and the most pellets per shot.
Only use big shot if it's for home defense. Not rabbit shot. Buck shot with "high brass" which is just a high-power powder charge is good.
Even birdshot at close range is effective and often times buckshot is not a good idea due to over penetration. The same is true of slugs. You need to adapt the ammo to your living conditions.
Make sure its magazine will hold many rounds. You can always buy a plug to limit the capacity for hunting, if you want to.
Don't get the tactical, black, pistol grip shotgun. You have no familiarity with shotguns, so get a nice wood one that would be at home in the field shooting birds. It'll be the same gun except for appearance. My reasoning is that you might actually want to go hunting, and you'll want to look the part when you do. But at home, the burglar won't care whether you use a hunting gun or a tactical one. Besides, there's something a little bit cool about defending your property with multi-purpose tools that just happen to be nearby.
If you are talking about shotguns that have only a pistol grip and no stock, I couldn't agree more. However, most HD shotguns are made of black polymers now and it would severely limit the viable choices if you insist on wood. Also, HD shotguns are specialty tools and there is no need to compromise effectiveness by getting a multi-purpose weapon.
My personal "arsenal" consists of a 9mm pistol and a 100+ year old 12-gauge side-by-side double. If you think you might need much more than that it's time to relocate."
If there is a possibility of a bad guy invading my house I want a hell of a lot more than two rounds from my HD shotgun. If I have moved to an area that this is not a concern, then I don't need a shotgun at all.