Every single thread turns into a buckshot vs birdshot debate.
If what I've learned from law enforcement and my shooting experience has taught me anything it's to use what WILL work. Not what might. I've been taught to take the maybe, might, or should work out of the equation when it comes to things as serious as using or carrying a gun. Especially in life or death situations. I'm not going to risk the life of myself, a brother in blue or a loved one because I didn't or couldn't do what was necessary. Especially when it comes to something as overrated as over penetration. For me it is a no brainer
Yes, I understand collateral damage but you have to be smart. Yes, a stray round MAY hit someone. It's happened in the past. But anything that is going to reliably stop an attacker, hopped on on drugs or not, is going to pass through a few layers of sheet rock. That's just the way it is. There's no magical round that will stop an intruder but stop immediately when hitting drywall. It's just not going to happen until there's a breakthrough in physics and engineering. In my opinion, not using the best of what is available and/or feasible is irresponsible and you're doing a disservice to those that you're trying to protect.
And to the people that say "But you wouldn't let me shoot you with a load of birdshot" or something to that effect. No, I wouldn't. But I also wouldn't let you shoot me with a rubber band. Does that make rubber bands an effective HD choice? I would think not.
Know the layout of your dwelling, who's sleeping where, what rooms are what and if there's any backstops. Do your own testing to see what works best.
CAN birdshot kill someone? Yes. But so can a pen to the neck. In most circumstances, birdshot creates a nasty surface wound which can be deceiving but what pellets do reach the vital organs don't do much damage. And bleeding out typically takes a long time.
Staggering loads is also not a good idea in my opinion (I.E birdshot, then buckshot). No matter what out of those two you shoot someone with, the police, prosecutor, and judge will treat that as using lethal force. And you may only be able to get one shot off to begin with so make it count. No warning shots, or hoping the birdshot will put someone down. Because like I said, no matter what is is considered using lethal force.
For me personally, I would go with nothing less than #4 buckshot. For an apartment, maybe T, BBB or F shot if you're nervous. Ideally, in a normal house #1 or 0BK is a good choice if you can find it although 00Bk and 000bk are more common and perfectly fine as well.
Birdshot is for hunting, practice or if you have absolutely nothing else available to you.
Just my .02
I may seem brash but when it comes to as serious of a topic as this I am going to be. It is not something to be taken lightly and I see far too many people that don't take this stuff seriously.