Wow, you notice all the different answers! I would NOT suggest birdshot at the front of your magazine.
I have chosen #1 Buck. I think 0-Buck, #1 Buck, and #4 Buck are all good choices.
000-Buck might penetrate too much, including slugs. But then, I've never shot a hole in my house's wall to "test".
Down home we used to go shooting where people would dump their garbage because they were too lazy to take it to the landfill. We shot up everything, nd not just to shoot up, but to see what would stop a certain caliber. a solid oak door from about 8 feet away birdshot will leave a fist sized hole in it. I use birdshot for my HD roundsin my shotguns and don't worry about stopping power.
what about a self defense situation inside a apartment and you have neihbors living all around you. what 12 gauge load would be best to stop the threat but wont harm anyone else.
There are none that would be 100%. You might want to consider moving a heavy piece of furniture to work as a backdrop. You know where your shotgun is, you know how the bad guy might approach, so put the furniture behind where the bad guy will be standing.
We have been down this road so often it needs repaved!
When you are deciding what size shot you need in a shotgun for ANY application you MUST consider the ballistics of ONLY ONE pellet as they separate once fired. For self defense you have to use a shot size which is able to defeat heavy clothing and still have the ability to reach the vital organs of your attacker. Really quite simple and just common sense for those of us who have both witnessed the effects of the shotgun on humans and acutely tested MANY different loads.
IF you are not concerned with over penetration any shot BB (.177) or larger will do the trick. BUT if you want to limit the penetration and still be effective on the attacker the choices are limited.
Stay with shot no smaller than BB and no larger than #4 buck and you will have the best of both worlds.
Birdshot is called birdshot for a reason! It lacks the penetration, even at close range, to stop a determined attacker. If you have a true 'contact' shot it will usually stop the fight, but even a few inches away from the target it will not.
Hope this helped (again)
An armed society is not always a polite society, but it is a free and safe society!
Self Defense is an absolute and natural right!
Keep your head down and your powder dry!