I like Tim Sundles take on it…
"The current 380 auto frangible ammo delivers a large amount of surface trauma but lacks serious penetration. For example, if you shot me or another sane man in the face with modern frangible 380 ammo, it would blow off a big portion of my cheek and send a few teeth down my throat, I would undoubtedly fall to the ground in shock and pain, but I would be very much alive and functional if I could get past the shock and pain as that frangible bullet would have stopped somewhere inside my face, never making it to my brain. However, if you shot a drugged up maniac in the face with that same frangible 380 ammo and blew half his cheek off, he would keep right on coming because he is insane and is not thinking like you or I. Plus, he is likely pain free and fear free and won't know that half his cheek is missing and if he did know, he would not care. So whatever 380 ammo you shoot him in the face with, had better go through his face and blow his brain stem out the back of his head, because only a CNS (central nervous system) hit with a 380 is going to stop him. Likewise, a torso hit to the sternum needs to penetrate deep enough to blow all the way through his spine to shut him down spontaneously. If you fail to shut him down instantly, you and your loved ones are going to have to find a way to survive while you wait for him to bleed out and pass out. The best chance of survival for you and your family is to shut down the attacker instantly."
So, hard cast flat nose for me.