Im going to go against others advice here, especially for your living arrangements. I live in an apartment as well, so I can relate to your concerns, and needs. In my stash of toys, I possess several 1911 45acp's, a pair of 40 S&W in tupperware guns, a 44 mag revolver, a model 1300 12ga, an sks, a pair of lever actions, bigbore and regular, and numerous bolt actions.
Of those listed, my primary weapon would be one of the pistols first, then the 44 mag(lower power loads mind you). All of my pistols have been ran thru, and I can vouge for their reliability. I want controlled fire, with single projectiles, in hollowpoint(read: low penetration). I have neighbors on each side, so I have two areas of no-fire zones in place, well three if you count the ceiling too. I already have known funnels, just like deer hunting. The front doorway goes down a hall, and you either go over, or around a sectional to access the rest of the apartment. Master bedroom has a hall to it as well. Window to the master bedroom is shielded with bushes. Sonic the hedgehog couldnt get thru those things. With that in mind, if I have a situation, I'm going to use those areas to my advantage, and let the guy either find Waldo, or leave. Your primary goal is to protect your life and loved one's lives, not hunt the perps down, though I wish we could at times! So if they run off, so be it. If they try to invade further, they have to go thru a funnel, and its a turkey shoot.
Now why do I say pistols? Pistols are meant to be a CQC weapon. Shotguns, or trenchguns, or zombie slayers, etc, would be intermediate range. Difference is, a pistol can be effectively fired at very close ranges, with minute of badguy accuracy, and not have the weapon extended out at all. Shotguns lack this. If you are caught offguard, and a perp is suddenly in your face, you have a weapon that can be leveraged from you, and used against you very quickly. Parkinglot shootouts would be more an ideal area for a scattergun IMO. The sks, and other rifles, too much penetration period. Out in the open space, a rifle may have its place, but we are in an apartment though. So I would recommend investing in a pistol of some kind and gain some practice, and leave the other long arms in the closet. Now if something happened tonite and you didnt have a pistol, the shotgun would be first choice, if it was reliable, then the sks. Remember, despite the internet rambos out there, if in a situation, fortify, or barricade yourself in to make a stand, call the dial-a-prayer(911)hotline, and wait for the police.