I've been in the military for a few years now, I've had a lot of range time with 9mms, A2s, M4s, mp5s, 249s and a lot of other fun stuff, and at three hundred yards I cant hit a sihloet with iron sights. Your first step should always be to assess what you are possibly up against and your environment, then determine the range of apropriate weapons and which one you are best suited to. If you might need to shoot past say 30 - 40 yards especially in the dark, a shotgun might not be your best choice, maybe an ar carbine with a night scope, if you live in an apartment lean towards a handgun (something like a 45 with soft ammo that doesn't penetrate much but decimates flesh) or shotgun. Personally I keep an off the shelf 870 shortbarrel with 6 #4 2 3/4 in the tube and one in on the button, and a good supply of (an assortment [slugs, buckshot, birdshot]) ammo nearby, and training,training, training.
Oh, and don't go out and get a 12 gauge if you can't handle it. If your thinking about it, go down to the local gun club and ask if you can try somebody's I'm sure they would be more than happy to accomidate you. pick something that you are comfortable with, but also pick something that fits your needs.
Practice practice practice!!!!!