So many folks get way too hung up on the FBI 12"-16" number. They forget that the FBI protocalls also call for going through car windshields, car doors, heavy clothing, and other barriers and still achieving those numbers.
As a non LEO, there are precious few times where firing through a car windshield would be a justified shooting.
In an article in another thread the author referenced a 158 gr. 357 fired through a carbine only penetrating 10 or so inches and failing this depth standard. Personally, I doubt a 158 gr. XTP fired at around 2000 fps. at 50 yards or less is going to fail to do the job simply because it failed to make the FBI last two inches.
Even from a handgun, a full energy dump into the agressor is still a full energy dump. Hitting the CNS, causing shock, hitting major arteries / veins, psycological stops, etc. are some of the things that "Stop" and attack.
22's have stopped attacks...but don't count on it. 45's have stopped attacks...but don't count on that either.