There was a study conducted on this site SOMEWHERE that I have been unable to find since. It was by an instructor if I remember correctly and in his study he cited documented LE and defense shootings.
He broke out the numbers based on how many shots it took to stop the threat across what types of guns/calibers, how many were actually "one shot stops", and their effective percentages. I remember that between handguns there was little to no difference. Shotguns outperformed BY FAR in terms of "one shot stops," but nearly all of the handgun shootings required multiple shots.
I've looked and looked and I have been unable to find it. It was several months back. If you find it post it back here please.
I think you are talking about Tom Givens of Rangemaster. He has done some research like this, I think.