Thanks to all for your responses.
I actually posted this to get your opinions for my daughter.
She is dead set in her belief that a 22 is all she needs for protection. She cites numerous statistics about how the 22 kills more people every year than any other round. I can't get her to move up to even a .380.
Try as I might, I can't make her understand that only a head shot from a 22 will instantly incapacitate an offender, and under the stress of being attacked she would be lucky to get one.
Now she's a great shot with her pistol on the range. She can reliably bust up the heads of her targets out to 25 feet. And she loves to prove it to me.
But I can't make her see that shooting at moving targets under extreme stress is another matter. She can't bank on a head shot. She'd be blessed to get a body shot. And a 22 body shot may not stop a determined bad guy.
"But dad, I've got 7 shots with my gun, I'll empty it in him."
And I'd like to repute that argument, but I think that she probably could empty a whole clip into an intruder. She's a pretty cool customer.
So again with the question I originally posted...would 3 or 4 shots from a 22 be as potent as 1 from a 9mm or 45?
Is my sharp shooting daughter wrong? If she can reliably unload 7 shots on an undesirable is she "relatively" safe?
Incidentally, I'm not as confident as she is. My home protection is a Mossberg 500 24" slug barrel loaded with quail shot, and my Ruger .223 Range Rifle.