Gun Toting Boeing Driver
Again it comes down to need and personal situation. And personal evaluation of risk. Tempered with information about history of defensive shootings and technical data regarding shot penetration effectiveness. A personal test of the situation a person is in is an excellent idea to gain insight as to what things might be like for HIS situation.I see that a number of videos have been posted on info for the op. I have not watched them but assume they provide good data. That being said if the op has the time, money, space and desire perhaps doing some of his own testing would answer his questions. Most, if not all, apartment walls are probably not over 6 inches thick based on my observations of construction. Perhaps building some wall sections and setting them at different distances, using different shot loads would give the answers he is looking for. I would guess that the average shooting distance in an apartment would not be much more than 10 to 12 feet. Walls would not consist of more than 2x4 studs, 5/8ths inch sheetrock and some fiberglass insulation. Some 3'x3' test panels should provide answers. Just my thoughts again.
While it has its limitations, the box o' truth has done some testing along these lines and might be a decent source to start with.
FOR ME, finer shot (birdshot) has too much of a checkered history when it comes to defensive shootings and as ranges go up (may or may not happen in a home, it well could most places I live) the chance of a non-stopping shot goes up fairly dramatically. Even buckshot loses its energy significantly (and drops) as the ranges go up. Would you NEED that in an apartment situation ? Dunno -- I dunno what kind of gunfight the guy might think himself as getting into.
I DO know the patterns of MY shotguns with various shot, and how they shoot with slugs. I get a pretty tight pattern inside 10 yards with most of the buckshot I might use (ranging from a saucer in diameter to perhaps a large dinner plate with buckshot; the birdshot spreads out a little wider covering a 2/3 IDPA target at 10 yards). So the spread really isn't all that great at HD ranges. Meaning that a miss is likely to be a miss with most of the pellets of whatever I'm shooting and a hit will also be a hit with most of what I'm shooting.
As such, while it's possible whatever I'm shooting COULD go through walls and hurt or kill someone, the chances of that happening aren't that great (keeping in mind that the slug shot during the Freeway church shooting DID penetrate through several walls and studs before coming to rest--and that there have been isolated cases of overpentration or bullet bounces hurting bystanders or others but these are exceedingly rare given the amount of defensive shootings). And a birdshot column at close range is going to go through walls just like anything else for the very same reason people are wanting to use it (namely that you have a dense highly packed shot column acting more like a slug--but with more pellets to disperse when it hits something). So for ME, I'd rather have buckshot (00,0,#1 or #2) because I can't count on birdshot getting through jackets and lots of fat at the longer ranges inside my dwelling and perhaps outside (depending on how things go); with buck I get more options (keeping in mind that a fight is a highly dynamic thing). Not to mention that I'm not going to have time to change ammo (nor in the heat of the moment probably remember what is stacked where in a double or pump if I'm using a mixed load of stuff).
The bottom line for ME is IF overpenetration IS the overriding concern (it's not), I probably don't want a shotgun. The best tool for me would be an M4 with potentially lighter bullet (which do tend to break up better than most bullets or buckshot).
Now, one thing that I HAVE thought about recently is changing shot in Dad's Taurus Judge/circuilt court (they're loaded federal 4-pellet buck and colt--the federal 4-pellet having some good results) mostly for outdoor snakes. The Winchester PDX load has 3 or 4 defense disks with BBs which rapidly spread out (good for varmints and snakes) and putting 1 or 2 followed by the buck/colts might be a better idea than what I've (he's) got. But what made me think of this was the time it took to run in to get the ol' coach gun over the mantle, change to fine shot, and shoot the copperhead that had run under the bushes (the pistol I had with me wouldn't work even IF I could have hit it due to potential collateral damage issues on the porch).