What do you need armor piercing ammo for?
You said: "Bear in mind this would be a time when there were Smilodons (saber tooth tiger), bear dogs or at least dire wolves, and other large dangerous predators, mastodons, bisons and other megafauna."
I'm hoping the armor piercing will go thru the skulls of some of those if I need to kill them in self defense. Trying to get the job done with minimal weight. I know, it's not a great hunting caliber, but don't want to carry enough .375 H&H to last a lifetime.
I guess I could just take a good .22LR for self defense and shoot them in both eyes so they couldn't find me.