Using a firearm in self defense is legally considered using lethal force, whether or not you are using birdshot, buckshot, or rubber "less lethal" options.
By using lethal force (firearm) in defense, you are essentially stating that you feel that your very life is in imminent danger, and if you don't use the lethal force, you are likely going to die.
In that context, the intent to wound, or "run off" an intruder insinuates that you aren't 100% in a situation where you are in imminent danger of losing your life. After all, if you were in imminent danger of death, you would use the most effective manner available to end the situation, and preserve your own life, right?
The intent with lethal force is to immediately incapacitate the aggressor, to force him to stop, by using a weapon.
There are of course, multiple ways of stopping a person, some more effective than others.
A CNS hit is the most effective, but can be hard to achieve.
A structural stop can stop the aggressor, but if he has a firearm, could still get a shot off, so it may not stop 100% of the threat.
A psychological stop is a great solution, but that involves aggressor having some rational thought and fear of what happens to them, either prior to, or after being shot, so because of that, it's not a reliable stopper. The psychological aspect is just one of the many reasons why Marshall and Sanow's "One stop shot" data can't be relied upon. There is no control, no indicator for when a psychological stop will occur, since there is no method of really knowing what's in an aggressor's head.
So what that leaves us with is the need to damage vital structures within the aggressor's body, to force him to stop through physical incapacitation. We don't want to rely on him "wanting" to stop, we want to force it.
In order to hit those vital structures, we need to be able to penetrate through to them, and create damage within them. That involves penetrating their clothes, skin, muscle, fat and bones prior to even reaching the vital organs.
Add in that an attacker may not be standing straight on towards you, and may have his arms in a position where they can block the path to their vital organs. Which means that now you need to penetrate through the aggressor's arms (skin, fat, muscle, bone, skin) prior to even hitting the body in front of his vital organs.
Add in the possibility of the aggressor being turned diagonally, and it's now easy to see why the experts in the wound ballistics field (who aren't trying to sell you anything) recommend a
minimum of 12" in calibrated gelatin.
I used to think like many others currently do, and used to keep my shotgun loaded with birdshot. I even once on this forum defended the practice of keeping "less lethal" options (rubber buck/slug), though it wasn't something I would do.
Through a significant amount of research, my views have evolved, and are now more inline with those of the experts in the field, and I will continue to follow their recommendations.
As such, my shotgun is loaded with 00 Buckshot, and is being replaced with Federal 00 Buckshot with flight control.
Use what you will, it's your life, and the life of your loved ones on the line.
For anyone that really wants an education in regards to wound ballistics, here are some links:
http://www.firearmstactical.com/
http://www.tacticalforums.com/ (requires registration, but is worth it. Look for posts by DocGKR (Doctor Gary Roberts) )
http://www.tacticalshotgun.ca/ballistics_shotgun.html