Here's my post on the subject from a related thread - the one about test shots on goats (seriously):
It has been my impression from reading this thread that animal tests record the results of a single shot - is that the case?
We are taught to shoot at the threat and to continue shooting at the threat until the threat is eliminated. Thus, the effect of a single shot is not unimportant but not necessarily conclusive as to the value of a particular caliber to a particular person.
Also, while a goat or person shot once may not have fallen down, the goat or person in at least some instances would be incapacitated to do any harm.
People are killed and not infrequently "stopped" every day from both single and multiple hand gun wounds.
A few days ago, local TV news showed a video of an attempted armed robbery of a convenience store. It was within about four miles of where I now sit. The person behind the counter grabbed a gun kept behind the counter and shot the perpetrator once, in the stomach and without any aiming apparent from the video. The perpetrator doubled over in apparent pain, and in that bent over posture ran (more or less) from the store. He died not far out of the door. Police arrived on the scene. The defending shooter was not charged with anything and was allowed to go home. The police said it was a very clearly justifiable shooting.
On the other side of the coin, a few weeks ago about five miles from where I sit now, goons with guns tried to rob an unarmed person who had just left a convenience store. He tried to escape by driving away. They chased after him in a car and shot him dead.
For my money, handguns are seriously deadly. They're easily carried and concealed. When the chips are down, I'd rather have one in my possession than not. Even a .22 is better than nothing, but a 9mm that I confidently can use is even better. Two shots are more deterrent than one, three are more deterrent that two, and so on, if you continue shooting (placement, placement, placement) until the threat is neutralized. YMMV