A .22 does not have much "knockdown" and for that reason I wouldn't even consider it for home defense. I would venture a guess that people have continued to function after multiple non-lethal shots from a .223 (roughly the same frontal area). My son mentioned another forum where this question was posed (it may have been your thread) where a guy asked "would you rather be punched in the chest 20 times by your 5-year old niece, or punched once in the chest by Mike Tyson?". Think about that!
Also, somebody else mentioned the fact that a high-velocity .22 will penetrate walls (a friend of mine accidentally fired my 10/22 inside my house
and it passed through two walls before exiting the house. The last exit hole did not indicate that the bullet had slowed much.) This is true. Do some testing at the range with layers of plywood, two-by-fours, sheetrock and what have you. You might be surprised at how deeply a .22 will penetrate. Think about the likelihood of collateral damage to innocents in other rooms or neighboring houses when you are firing up to 100 shots under high-stress out of your .22. I would guess that many of us do not have that type of training to shoot with a high degree of accuracy under those conditions.
Pump action shotgun (any size gauge) for a couple of reasons. First, even though I would not hesitate to shoot somebody breaking into my home, I never want to have to do that. The sound of a pump action chambering a round is enough to make most bad guys wet their pants and give up (RACK!-RACK!
"Stop pissing on my floor, A-Hole!"
). It also would give an innocent person who has mistakenly entered the wrong house (I admit it, living in a rural setting the door sometimes does not get locked at night) to scream out "don't shoot!" and you can turn on the lights, evaluate the situation and then decide if he is a bad guy who deserves to be shot in the chest with 00 buckshot.
My HD gun is a .357 double-action revolver loaded with Hydra Shok .38 Specials (no need for the magnum loads at close quarters, plus they are a bit quieter.) I also have a speed loader if I need it for multiple bad guys. Here in the People's Republic of California if one were to reload and shoot a bad guy again it would not bode well for you in court.
Another piece of advice I have received from several Sheriff deputies is to make sure the BG is not breathing, is armed (preferably his gun has been fired into the house at least once) and he is inside the house when they get there. Often makes for an open and shut case when (in CA) it comes to that.