Nobody wants to argue with a man with a shotgun.
That's mostly true...because most of the time, they assume he's loading buckshot. The times when they are willing to argue with him, he's going to need that buckshot, because they are in an altered mental state.
Azar,
Do you not understand the concept of inertia? Little pellets don't travel as far in air because of wind resistance. They don't maintain velocity against friction. The same thing happens when they impact flesh/clothing. To penetrate and hit vital organs, it has to have a cetain amount of inertia. Once the shot has left the shot cup, each pellet has its own inertia. At point blank range, if it has enough power to penetrate enough human flesh to reach vitals, it's going to go through a wall and hurt someone on the other side. I agree that slugs are overkill for HD, but birdshot isn't going to reach vitals and make a big enough hole to be effective. This has been tested time and again. There's a GOOD REASON that the FBI and IWBC reccomend nothing smaller than #1 buckshot for self defense out of a shotgun. Anything less won't penetrate vitals to get a quick stop. Make no mistake: That birdshot can be lethal, but it isn't going to incapacitate someone pumped up on adrenaline or drugs fast enough to matter.
If it can penetrate and stop a person, it's going to penetrate a wall and be dangerous on the other side. If it can't penetrate a person, or most of one, it isn't going to be effective (especially figuring the increasing amounts of fat most Americans are carrying around). You can't escape that.
And please don't launch into the "birdshot at close range is the same as buckshot or a lead slug" nonsense. Here's a non-lethal illustration of the same principle: Throw a handful of sand at someone's head at arm's reach (travels 2 inches from hand to target). Then throw a handful of sandstone at the same distance. Notice the different effect (one makes them spit out sand, the other requires an ice pack and band aids). Same material, but one sticks together in one solid piece, the other is lots of little bitty pieces that each have to overcome air or flesh resistance. If this wasn't true, at home defense range, the birdshot would be just as effective as a slug. Why isn't this true? Because the larger projectile penetrates more; it retains the entire projectile's inertia as it passes through it's target.
So, understanding this basic principle, you have to get projectiles large enough to penetrate vital organs, without passing through the next ten walls or failing to penetrate the skin and first layer or two of muscle. Answer? Somewhere between #4 and 000 buckshot, depending on who you talk to. At less than two meters, goose loads MIGHT be effective, but then you still run the risk of penetrating walls.
Answer? Practice, and don't miss. Even better, plan out ahead of time what your most defensible position is, and create one lane of fire with a solid backstop. Anything else you are playing with a pissed off attacker who hasn't been stopped, or potentially dead neighbors.
I've heard about and read about far too many people shot at home defense range with birdshot and still been standing to put any trust in that as a defense load. It creates a vicious crater-type wound, but it's shallow. Our vice president shot an elderly guy in the face at about 18 yards (the official range estimate must politely be called fantastical spin/fiction), and the guy lived. I don't have ANY faith that it would stop a coked up/drunk/pissed-off fatbody/weightlifter at 10 yards (and I have a clear line of sight 12 yards long down the hallway and into a bedroom as I am typing this). I've done enough testing on milk jugs to know that birdshot even at close range just won't get more than about 6 inches of penetration in flesh.
One example from the box of truth via AR-15.com (granted, hearsay, but better hearsay than "theory")
A friend of AR15.com sends this:
"I saw a gunshot victim, about 5' 10" and 200 lbs, taken to the operating room with a shotgun wound to the chest. He was shot at a range of six feet at a distance of just over the pectoralis muscle. He was sitting on his front porch and walked to the ambulance. We explored the chest after x-rays were taken. The ER doc had said 'buckshot' wound, but this was obviously not accurate.
It was # 6 shot. There was a crater in the skin over an inch in diameter. When the shot hit the level of the ribs, it spread out about five inches. There was ONE pellet that had passed between the ribs and entered the pericardium, but not damaged the heart at all. As you say, 'use birdshot for little birds.'"
See also:
http://www.theboxotruth.com/docs/bot14_4.htm
Penetrated only 1 sheet or drywall at what appears to be less than 10 yards, probably around five yards. That's not going to stop anybody except a two-year old.