The FBI's "recommended penetration" depth takes into account shooting someone in their outstretched arm, as if they were aiming a gun at you, and still hitting a vital organ. It's strange that so many perpetrators were stopped with "inadequate" penetration before this study. Perhaps the bad guys didn't read it.....
Expanding ammunition put the lesser calibers on the map as defense cartridges many years ago. They generally work.
I can not understand why this one "study" is touted so highly by the Internet commandos.
If you happen to hit a bad guy in the outstretched arm, he isn't aware of the FBI's recommendation, he doesn't feel pain, he still has enough muscle control to fire his gun, he hasn't already turned and run away, and is insanely determined to keep coming you might need all that penetration.
Or not.