There seems to be an enduring myth that it's a good idea to shoot somebody "just a little" to discourage them from advancing. Then, if they don't have the courtesy to get sufficiently scared off & keep coming, "let 'em have it with the real thing".
This is about as realistic as trying to shoot the gun/knife out of their hand, just "winging" them, firing a warning shot straight up to let 'em you're "serious", and "I don't really want to hurt anybody, I just want to scare them away".
The legal justification for using birdshot on someone in a defensive situation is exactly the same as it is with buckshot.
If you're justified legally in shooting at all, why endanger your life by deliberately using ineffective tiny shot charges that simply don't disable or stop an advancing threat to begin with?
If you have to shoot, shoot to survive, not to **** somebody off.
Other considerations are they're frequently not reliable in autopistols (I tested them in all calibers available through over 20 rifles, revolvers, and autos 12 or 13 years back), and if you use a revolver you're giving up one third of your onboard capacity (more in a five-shot snub) to two largely worthless shots in an enclosed (CLOSE!) space where you almost certainly won't have time to reload before the guy (or guys) can be right on top of you.
It remains a fact that overpenetration is one downside of living in an apartment, and it remains a fact that if you try too hard to ensure no wall penetration by using anemic loads and tiny shot charges, you reduce your own defenses.
I'm not being unsympathetic to anybody on either side of the wall, just addressing the handgun shotshell & "loaded down" issues.
To use a firearm in an apartment, with its attendant risks, is a choice Rod will have to make for himself.
My concern is that it's an educated choice.
Denis