As part of doing some IDPA stage design, I databased every incident in "The Armed Citizen" for 1998-2000; over 300 incidents. Entries for type of gun carried by citizen, were shots fired, how many, results, etc. Granted there are methodological problems with this data sample, but it's not a perfect world.
Bottom line - have a gun, any gun, and be willing to use it. When shots were fired, the mean number of shots fired was 2, median 2, mode 1. Most common caliber used by citizens was .38/.357, second most popular was .22/.25. "Heavy caliber" handguns were pretty far down the list. The only cases I could find where a criminal who was shot with any caliber weapon continued to advance were psychotic domestic violence situations. Robbers, home invaders, rapists, etc. seemed to feel that being shot once or twice (usually once) was quite enough, thank you, and either fled or expired.
FWIW. I think the creativity of the human imagination is quite amazing, especially when questions of what is adequate weaponry come into play. As one poster to this discussion said, if you have to pull a pistol, the weapon you will be wishing for will not be some slightly larger pathetic popgun handgun, because any handgun is a mousegun. Most likely visions of a shotgun or belt fed GPMG will be dancing in your head, unless you left your hogleg at home, in which case you will be thinking "I wish I had a gun, any gun."