Guardian,My opinion remains the same...go with buckshot, whether it's 4, 3, 1, or 00 (I've yet to figure out any real defensive advantage to 000).Living in an apartment poses some of the same problems one encounters in a single family dwelling. I have 4 bedrooms on the top level of our home, containing Mrs Ripley, 3 kids, and yours truly when I'm not in the dog house. Course of Action #1 for bumps in the night is to first opt for the two condition 1 handguns (and the keys to one of the long gun safes) in a quick access safe next to the master bed. My "bedside" handgun is a Glock23 with M3 tac-light and tritium sights, so that (if at all possible) I can carry the smallest child while herding the other two back to the master bathroom (which serves as a safe room). I can still, onehanded, ID any intruders and shoot if necessary. Meanwhile Mrs. Ripley has a second handgun and opens the safe containing my 870.Once back in the master bedroom I'll shift to the 870 with a loading of 00 buck (2 3/4' to reduce the likelihood of "short stroking" a 3" shell). After shooting A LOT of different brands, what performs best in my barrel after having it overbored and the forcing cone lenghtened is ESTATE 8-Ball. "Best" is defined by a consistent pattern and ability to keep all the pellets in the torso out to extended ranges (with my combo it's in excess of 35 yards). The only reason I even care about extended ranges is the tiny chance I would find myself having to engage someone outside firing into my home. I'm lucky that the cheap stuff is what performs best in my particular barrel. After giving you more info than you asked for, let me answer your original question. If I can't gather the kids I essentially have your apartment situation. If the problem is downstairs and all family members upstairs, I simply cover the stairway and await the calvary. If a bad guy is upstairs and I have to shoot, the best approach (in my situation) is to take a knee and fire upwards, just as one should do in a crowded situation on the street. My background has caused me to "get small" almost instinctively, so going low and staying low is part of my routine. Any pellets that miss an assailant will continue upwards and pass "over" the wife and kids in other rooms. This negates concerns about penetration from the 00 buck.Obviously this approach will not work for everyone. My best advice is to predetermine "safe" firing lanes and directions. I'd loosely defined a "safe" lane or direction as one that minimizes risk due to structural penetration. This requires abandoning any urge to "prowl and growl", instead requiring you to pick a piece of ground that allows you to cover threat areas, but still uses a "safe" firing lane. A defensive firing position real close to where you acquire the shotgun is best. Think about solid backstops like major appliances and bookcases. Perhaps adjoining utility rooms will work. Maybe a dumpster beyond an exterior wall. You can even position bookcases to your benefit, either to act as a backstop or as cover for you. THE MOST IMPORTANT CONSIDERATION FOR AVOIDING STRUCTURAL PENETRATION IS THE ABILITY TO HIT RELIABLY. Shoot enough, from realistic positions, to hit. Regardless, stick with something that can penetrate to major blood vessels and vital organs. Preferably something that has the word "buck" in the load description. To minimize penetration if everything goes wrong, or if you're in that middle unit on a middle floor, my advice is to go with 4 Buck. Stay safe!